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Abstract:

A radio base station apparatus for performing radio communication with a
mobile terminal apparatus, the radio base station apparatus including: a
forwarding data determination unit which determines forwarding data which
is to be forwarded to a handover destination radio base station
apparatus, based on the presence or absence of retransmission of data to
the mobile terminal apparatus; and a data forwarding processing unit
which forwards the determined forwarding data to the handover destination
radio base station apparatus.

Claims:

1. A radio base station apparatus for performing radio communication with
a mobile terminal apparatus, the radio base station apparatus comprising:
a forwarding data determination unit which determines forwarding data
which is to be forwarded to a handover destination radio base station
apparatus, based on the presence or absence of retransmission of data to
the mobile terminal apparatus; and a data forwarding processing unit
which forwards the determined forwarding data to the handover destination
radio base station apparatus.

2. The radio base station apparatus according to claim 1, further
comprising a memory unit which stores data to be transmitted to the
mobile terminal apparatus, wherein the forwarding data determination unit
determines the forwarding data when handover is decided in respect of the
mobile terminal apparatus and the data is stored in the memory unit.

3. The radio base station apparatus according to claim 1, further
comprising a memory unit which stores data to be transmitted to the
mobile terminal apparatus, wherein the forwarding data determination unit
determines a first data group including data for which a retransmission
is performed and a second data group which is stored in the memory unit
as the forwarding data, when the retransmission of the data is performed,
and determines the second data group which is stored in the memory unit
as the forwarding data, when the retransmission of the data does not be
performed, and the data forwarding processing unit forwards the first
data group and the second data group to the handover destination radio
base station apparatus when the retransmission of the data is performed,
and forwards the second data group to the handover destination radio base
station apparatus when the retransmission of the data does not be
performed.

4. The radio base station apparatus according to claim 1, wherein the
forwarding data determination unit determines the forwarding data based
on the presence or absence of the retransmission of the data during a
monitoring period.

5. The radio base station apparatus according to claim 1, wherein the
forwarding data determination unit determines a sequence number of the
data at which transmission from the handover destination radio base
station apparatus to the mobile terminal apparatus is to be started, when
a data group having one or a plurality of data is determined to be the
forwarding data, and the data forwarding processing unit notifies the
determined sequence number to the handover destination radio base station
apparatus.

6. The radio base station apparatus according to claim 1, wherein the
forwarding data determination unit calculates a retransmission occurrence
rate based on the presence or absence of data retransmission, for each
handover destination radio base station apparatus, and determines the
forwarding data based on a radio quality corresponding to the
retransmission occurrence rate.

7. The radio base station apparatus according to claim 6, further
comprising a memory unit which stores data to be transmitted to the
mobile terminal apparatus, wherein the forwarding data determination unit
judges the retransmission occurrence rate, which is equal to or greater
than a threshold value, as a first radio quality and judges the
retransmission occurrence rate, which is lower than the threshold value,
to be a second radio quality, and determines, to be the forwarding data,
a first data group including data which is transmitted to the mobile
terminal apparatus and a second data group which is stored in the memory
unit when judged that the first radio quality takes effect, and
determines, to be the forwarding data, the second data group which is
stored in the memory unit when judged that the second radio quality takes
effect, and the data forwarding processing unit forwards the first data
group and the second data group to the handover destination radio base
station apparatus, when the radio quality is judged to be the first radio
quality, and forwards the second data group to the handover destination
radio base station apparatus when the radio quality is judged to be the
second radio quality.

8. The radio base station apparatus according to claim 6, wherein the
forwarding data determination unit determines a sequence number of the
data at which transmission from the handover destination radio base
station apparatus to the mobile terminal apparatus is to be started, when
a data group having one or a plurality of data elements is determined to
be the forwarding data, and the data forwarding processing unit reports
the determined sequence number to the handover destination radio base
station apparatus.

9. A radio base station apparatus for performing radio communication with
a mobile terminal apparatus, the radio base station apparatus comprising:
a forwarding data determination unit which determines forwarding data to
be forwarded to a handover destination radio base station apparatus,
based on a radio quality between the radio base station apparatus and the
mobile terminal apparatus; and a data forwarding processing unit which
forwards the determined forwarding data to the handover destination radio
base station apparatus.

10. The radio base station apparatus according to claim 9, further
comprising a memory unit which stores data to be transmitted to the
mobile terminal apparatus, wherein the forwarding data determination unit
determines the forwarding data when execution of handover is decided in
respect of the mobile terminal apparatus and the data is stored in the
memory unit.

11. The radio base station apparatus according to claim 9, wherein the
forwarding data determination unit calculates a retransmission occurrence
rate based on the presence or absence of a retransmission of the data,
for each handover destination radio base station apparatus, and
determines the forwarding data based on a radio quality which corresponds
to the retransmission occurrence rate.

12. The radio base station apparatus according to claim 11, further
comprising a memory unit which stores data to be transmitted to the
mobile terminal apparatus, wherein the forwarding data determination unit
judges the radio quality to be a first radio quality when the
retransmission occurrence rate is equal to or greater than a threshold
value and judges the radio quality to be a second radio quality when the
retransmission occurrence rate is lower than the threshold value, and
determines a first data group including data which is transmitted to the
mobile terminal apparatus and a second data group which is stored in the
memory unit, as the forwarding data when judged that the first radio
quality takes effect, and determines the second data group which is
stored in the memory unit, as the forwarding data, when judged that the
second radio quality takes effect, and the data forwarding processing
unit forwards the first data group and the second data group to the
handover destination radio base station apparatus, when the radio quality
is judged to be the first radio quality, and forwards the second data
group to the handover destination radio base station apparatus when the
radio quality is judged to be the second radio quality.

13. The radio base station apparatus according to claim 11, wherein the
forwarding data determination unit determines a sequence number of the
data at which transmission from the handover destination radio base
station apparatus to the mobile terminal apparatus is to be started, when
a data group having one or a plurality of data elements is determined to
be the forwarding data, and the data forwarding processing unit reports
the determined sequence number to the handover destination radio base
station apparatus.

14. The radio base station apparatus according to claim 9, further
comprising a memory unit which stores data to be transmitted to the
mobile terminal apparatus, wherein the forwarding data determination unit
judges the radio quality to be a third radio quality when the radio
quality measured in the mobile terminal apparatus or the radio base
station apparatus is equal to or greater than a threshold value and
judges the radio quality to be a fourth radio quality when the measured
radio quality is lower than the threshold value, and determines a third
data group including data which is transmitted to the mobile terminal
apparatus and a fourth data group which is stored in the memory unit, as
the forwarding data when judged that the third radio quality takes
effect, and determines the fourth data group which is stored in the
memory unit, as the forwarding data when judged that the fourth radio
quality takes effect, and the data forwarding processing unit forwards
the third data group and the fourth data group to the handover
destination radio base station apparatus, when the radio quality is
judged to be the third radio quality, and forwards the fourth data group
to the handover destination radio base station apparatus, when the radio
quality is judged to be the fourth radio quality.

15. A radio base station apparatus for performing radio communication
with a mobile terminal apparatus, the radio base station apparatus
comprising: a forwarding data determination unit which determines
forwarding data to be forwarded to a handover destination radio base
station apparatus, based on the presence or absence of a data
retransmission to the mobile terminal apparatus and a radio quality
between radio base station apparatus and the mobile terminal apparatus;
and a data forwarding processing unit which forwards the determined
forwarding data to the handover destination radio base station apparatus.

16. A data forwarding method for a radio base station apparatus for
performing radio communication with a mobile terminal apparatus, the
method comprising: determining forwarding data to be forwarded to a
handover destination radio base station apparatus, based on the presence
or absence of a data retransmission to the mobile terminal apparatus; and
forwarding the determined forwarding data to the handover destination
radio base station apparatus.

17. A data forwarding method for a radio base station apparatus for
performing radio communication with a mobile terminal apparatus, the
method comprising: determining forwarding data to be forwarded to a
handover destination radio base station apparatus, based on a radio
quality between the radio base station apparatus and the mobile terminal
apparatus; and forwarding the determined forwarding data to the handover
destination radio base station apparatus.

18. A data forwarding method for a radio base station apparatus for
performing radio communication with a mobile terminal apparatus, the
method comprising: determining forwarding data to be forwarded to a
handover destination radio base station apparatus, based on the presence
or absence of a data retransmission to the mobile terminal apparatus and
a radio quality between the radio base station apparatus and the mobile
terminal apparatus; and forwarding the determined forwarding data to the
handover destination radio base station apparatus.

19. The radio base station apparatus according to claim 9, wherein the
forwarding data determination unit determines the forwarding data, based
on a first radio quality which corresponds to a retransmission occurrence
rate in the handover destination radio base station apparatus that is
calculated based on the presence or absence of data retransmission, and a
second radio quality which is measured in the mobile terminal apparatus
or the radio base station apparatus.

20. The radio base station apparatus according to claim 1, further
comprising a memory unit which stores data to be transmitted to the
mobile terminal apparatus, wherein the forwarding data determination unit
transmits a first data of data included in the first data group which is
stored in the memory unit, to the mobile terminal apparatus when
execution of handover of the mobile terminal apparatus to the handover
destination radio base station apparatus is decided, and determines the
forwarding data based on the presence or absence of a retransmission in
respect of the first data, and based on whether or not transmission of a
second data included in the first data is possible, when the second data
does not be transmitted to the mobile terminal apparatus.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priority
of the prior Japanese Patent Application No. 2011-058940, filed on Mar.
17, 2011, and the Japanese Patent Application No. 2011-200450, filed on
Sep. 14, 2011, the entire contents of which are incorporated herein by
reference.

FIELD

[0002] The embodiments discussed herein are related to a radio base
station apparatus, and a data forwarding method in the radio base station
apparatus.

BACKGROUND

[0003] At present, radio communication systems, such as mobile telephone
systems or radio LANs (Local Area Networks), and the like, are used
widely. Furthermore, in the field of radio communication, there are
continuous discussions about next-generation communication technology in
order to further improve communication speed and communication capacity.
For example, the 3GPP (3rd Generation Partnership Project), which is one
standardization group, is proposed a radio communication system called
LTE (Long Term Evolution) and a radio communication system known as LTE-A
(Long Term Evolution-Advanced), which is a development of LTE.

[0004] In the radio communication system, there is technology known as a
handover. The handover is a technology for switching the radio base
station apparatus (Evolved UTRAN NodeB (ENB), hereinafter called "base
station") to which a mobile terminal apparatus (Mobile Station,
hereinafter called "terminal") is connected. By this means, the terminal
is able to perform a radio communication in a continuous fashion, by
switching connection to another base station, when the received radio
wave becomes weaker than a prescribed value.

[0005] In the handover, there are cases where data is not transmitted to
the terminal from a handover source base station and the data is
forwarded to a handover destination base station. By means of the
forwarding, for example, the terminal is able to continue receiving data
in a continuous fashion from the handover destination base station, when
the terminal is switched connection to the handover destination base
station.

[0006] There are, for instance, two methods for forwarding data between
base stations by the handover. The first method is a method which
forwards data that does not transmit to the terminal as forwarding data,
to the handover destination base station (this method is called "mode 1"
below). Furthermore, the second method is a method which forwards data
for which an Ack signal (or a reception confirmation notification) does
not receive from the terminal, as forwarding data, to the handover
destination base station (this method is called "mode 2" below).

[0007]FIG. 29 and FIG. 30 are sequence diagrams which respectively
illustrate operational examples of the forwarding method in the mode 1
and mode 2. In both of these examples, the terminal UE is connected to
the serving base station S-ENB and performs handover to a target base
station T-ENB as a handover destination. Furthermore, in both of these
cases, it is supposed that the serving base station S-ENB receives three
SDUs (Service Data Units, SDU-A to SDU-C) from a gateway GW, and of these
transmits the data in SDU-A to the terminal UE. SDU-A includes PDUs
(Protocol Data Units) having sequence numbers SN1 to SN6, and SDU-B
includes PDUs having sequence numbers SN7 to SN13. In both of the
examples, it is supposed that the serving base station S-ENB transmits
PDUs having sequence numbers SN1 to SN6 to the terminal UE, and receives
Ack signals relating to the PDUs having sequence numbers SN1 to SN3.

[0008] Under circumstances such as these, in mode 1, untransmission data
is set as forwarding data regardless of the presence or absence of an Ack
signal, and therefore in the example in FIG. 29, the PDUs from SN7
onwards which belong to the SDU-B are set as forwarding data. In this
case, the serving base station S-ENB reports the sequence number SN7 to
the target base station T-ENB (S104), and forwards SDU-B and SDU-C
(S105). The base station T-ENB transmits PDUs from sequence number SN7
onwards to the terminal UE (S106).

[0009] On the other hand, in mode 2, the data for which an Ack signal does
not be received is set as forwarding data, and therefore in the example
in FIG. 30, the PDUs from sequence number SN4 onwards are set as
forwarding data. In this case, the serving base station S-ENB reports the
sequence number SN4 to the target base station T-ENB (S110), and forwards
the data of SDU-A, SDU-B and SDU-C including the PDU having sequence
number SN4 (S111). The base station T-ENB transmits the PDUs from
sequence number SN4 onwards, to the terminal UE (S106). [0010] Patent
Document 1: Japanese Laid-open Patent Publication No. 2009-267840 [0011]
Patent Document 2: Japanese Laid-open Patent Publication No. 2000-69522
[0012] Patent Document 3: Japanese Laid-open Paten Publication No.
2006-217219 [0013] Patent Document 4: Japanese Laid-open Patent
Publication No. 2007-96968

[0014] However, in the case of mode 1, data which the terminal UE may not
be able to receive is not forwarded from the serving base station S-ENB
to the target base station T-ENB, and hence there are cases where loss of
data occurs at the terminal UE.

[0015] For example, in the example in FIG. 29, the serving base station
S-ENB does not confirm reception of Ack signals in respect of the PDUs
having sequence numbers SN4 to SN6. Consequently, there is a possibility
that the terminal UE does not be able to receive the PDUs having sequence
numbers SN4 to SN6. In a situation such as this, even if the serving base
station S-ENB forwards the sequence number SN7 onwards, it does not
forward the PDUs having sequence numbers SN4 to SN6 which may possibly
not be received by the terminal UE, and therefore the terminal UE is not
able to receive the PDUs having sequence numbers SN4 to SN6.
Consequently, in the case of mode 1, there are situations were the PDUs
having sequence numbers SN4 to SN5 are lost at the terminal UE.

[0016] On the other hand, in the case of mode 2, there are situations
where data for which the terminal UE may transmits an Ack signal is
forwarded from the serving base station S-ENB to the target base station
T-ENB. Accordingly, there are cases where the base station T-ENB
transmits data to the terminal UE in a duplicated fashion, and the
terminal UE receives the data in a duplicated fashion.

[0017] For instance, in the example in FIG. 30, since the serving base
station S-ENB does not receive an Ack signal in respect of the PDUs
having sequence numbers SN4 to SN6, then the serving base station S-ENB
forwards the PDUs having sequence numbers from SN4 onwards, to the target
base station T-ENB. In cases such as these, for example, the terminal UE
may receives the PDUs having sequence numbers SN4 to SN6 correctly and
transmitted Ack signals. There are cases where the serving base station
S-ENB makes a handover decision and forwards data before confirming
reception of the Ack signals. In a situation such as this, even though
the serving base station S-ENB forwards the PDUs having sequence numbers
from SN4 onwards to the target base station T-ENB, the PDUs having
sequence numbers SN4 to SN6 which may receive by the terminal UE, are
also forwarded. Therefore, the base station T-ENB transmits the PDUs
having sequence numbers SN4 to SN6 in duplicated fashion to the terminal
UE, and the terminal UE also receives the PDUs having sequence numbers
SN4 to SN6 in duplicated fashion. If the terminal UE receives data in a
duplicated fashion, then the terminal needs to perform unnecessary
processing, such as processing for discarding this data, and the like.

SUMMARY

[0018] According to an aspect of the invention, a radio base station
apparatus for performing radio communication with a mobile terminal
apparatus, the radio base station apparatus including: a forwarding data
determination unit which determines forwarding data which is to be
forwarded to a handover destination radio base station apparatus, based
on the presence or absence of retransmission of data to the mobile
terminal apparatus; and a data forwarding processing unit which forwards
the determined forwarding data to the handover destination radio base
station apparatus.

[0019] The object and advantages of the invention will be realized and
attained by means of the elements and combinations particularly pointed
out in the claims.

[0020] It is to be understood that both the foregoing general description
and the following detailed description are exemplary and explanatory and
are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

[0021]FIG. 1 is a diagram illustrating an example of the composition of a
radio communication system;

[0022]FIG. 2 is a diagram illustrating an example of the composition of a
radio communication system;

[0023]FIG. 3 is a diagram illustrating an example of the composition of a
radio base station apparatus;

[0024]FIG. 4 is a diagram illustrating an example of the composition of a
mobile terminal apparatus;

[0025] FIG. 5 is a sequence diagram illustrating an operational example in
a radio communication system;

[0026]FIG. 6 is a diagram illustrating an example of a retransmission
information table;

[0027]FIG. 7 is a sequence diagram illustrating an operational example in
a radio communication system;

[0028]FIG. 8 is a sequence diagram illustrating an operational example in
a radio communication system;

[0057]FIG. 37 is a flowchart illustrating an operational example of a
forwarding data determination process;

[0058]FIG. 38 is a sequence diagram illustrating an operational example
in a radio communication system;

[0059]FIG. 39 is a flowchart illustrating an operational example of a
forwarding data determination process;

[0060] FIG. 40 is a sequence diagram illustrating an operational example
in a radio communication system;

[0061]FIG. 41 is a sequence diagram illustrating an operational example
in a radio communication system;

[0062]FIG. 42 is a flowchart illustrating an operational example of a
forwarding data determination process;

[0063]FIG. 43 is a diagram illustrating an example of forwarding data;

[0064]FIG. 44 is a sequence diagram illustrating an operational example
in a radio communication system;

[0065]FIG. 45 is a sequence diagram illustrating an operational example
in a radio communication system;

[0066]FIG. 46 is a sequence diagram illustrating an operational example
in a radio communication system;

[0067]FIG. 47 is a sequence diagram illustrating an operational example
in a radio communication system;

[0068]FIG. 48 is a sequence diagram illustrating an operational example
in a radio communication system;

[0069]FIG. 49 is a sequence diagram illustrating an operational example
in a radio communication system;

[0070] FIG. 50 is a sequence diagram illustrating an operational example
in a radio communication system;

[0071]FIG. 51 is a sequence diagram illustrating an operational example
in a radio communication system;

[0072]FIG. 52 is a sequence diagram illustrating an operational example
in a radio communication system;

[0073]FIG. 53 is a sequence diagram illustrating an operational example
in a radio communication system;

[0074]FIG. 54 is a sequence diagram illustrating an operational example
in a radio communication system;

[0075]FIG. 55 is a sequence diagram illustrating an operational example
in a radio communication system; and

[0076]FIG. 56 is a diagram illustrating an example of forwarding data.

DESCRIPTION OF EMBODIMENTS

[0077] Below, embodiments of the present invention will be described.

First Embodiment

[0078] To begin, a first embodiment of the invention will be described.
FIG. 1 is a diagram illustrating an example of the composition of a radio
communication system 10 according to a first embodiment. The radio
communication system 10 includes the radio base station apparatuses 200a
and 200b and a mobile terminal apparatus 100.

[0079] The mobile terminal apparatus 100 is able to perform radio
communication with the radio base station apparatuses 200a and 200b, and
is able to switch a radio connection from the handover source radio base
station apparatus 200a to the handover destination radio base station
apparatus 200b.

[0080] The radio base station apparatus 200a includes a forwarding data
determination unit 270 and a data forwarding processing unit 280.

[0081] The forwarding data determination unit 270 can determine forwarding
data which is to be forwarded to the handover destination radio base
station apparatus 200b, based on the presence or absence of a data
retransmission to the mobile terminal apparatus 100. Furthermore, the
forwarding data determination unit 270 can also determine forwarding data
to be forwarded to the handover destination radio base station apparatus
200b, based on the radio quality in relation to the mobile terminal
apparatus 100. Moreover, the forwarding data determination unit 270 is
also able to determine forwarding data to be forwarded to the handover
destination radio base station apparatus 200b, based on the presence or
absence of the data retransmission to the mobile terminal apparatus 100
and the radio quality in relation to the mobile terminal apparatus 100.

[0082] The data forwarding processing unit 280 is able to forward the
determined forwarding data to the handover destination radio base station
apparatus 200b.

[0083] In this way, the radio base station apparatus 200a determines
forwarding data to be forwarded to the handover destination radio base
station apparatus 200b, based on the presence or absence of the data
retransmission to the mobile terminal apparatus 100. Consequently, for
example, when the data retransmission is performed, the data which is the
object of the retransmission (retransmission data) is set as forwarding
data, and therefore the mobile terminal apparatus 100 is also able to
receive the retransmission data from the handover destination radio base
station apparatus 200b and loss of data can be prevented. Furthermore, if
the data retransmission does not be performed, for instance, then data
which does not be transmitted from the radio base station apparatus 200a
to the mobile terminal apparatus 100 is set as forwarding data, and it is
possible to prevent duplicated transmission of data by the radio base
station apparatus 200b and duplicated reception of data by the mobile
terminal apparatus 100.

[0084] Furthermore, the radio base station apparatus 200a determines
forwarding data to be forwarded to the handover destination radio base
station apparatus 200b, based on the radio quality in relation to the
mobile terminal apparatus 100. Consequently, if the radio quality is not
good, for example, then data which is transmitted from the radio base
station apparatus 200a is also set as forwarding data, and therefore it
is possible to prevent loss of data, because the mobile terminal
apparatus 100 receives this data from the handover destination radio base
station apparatus 200b. Furthermore, if the radio quality is good, for
example, then data which does not be transmitted from the radio base
station apparatus 200a to the mobile terminal apparatus 100 is set as
forwarding data, and thus it is possible to prevent duplicated
transmission of data by the radio base station apparatus 200b and
duplicated reception of data by the mobile terminal apparatus 100.

[0085] Moreover, the radio base station apparatus 200a determines
forwarding data to be forwarded to the handover destination radio base
station apparatus 200b, based on the presence or absence of the data
retransmission to the mobile terminal apparatus 100 and the radio quality
in relation to the mobile terminal apparatus 100. Consequently, if the
data retransmission does not be performed and the radio quality is good,
for example, then data which does not be transmitted from the radio base
station apparatus 200a to the mobile terminal apparatus 100 is set as
forwarding data. Accordingly, it is possible to prevent duplicated
transmission of data by the radio base station apparatus 200b and
duplicated reception of data by the mobile terminal apparatus 100.
Furthermore, if the data retransmission does not be performed but the
radio quality is not good, or if the data retransmission is performed,
for example, then the data transmitted from the radio base station
apparatus 200a is set as forwarding data. Accordingly, the mobile
terminal apparatus 100 receives this data from the handover destination
radio base station apparatus 200b and hence it is possible to prevent
loss of data.

Second Embodiment

Example of General Composition

[0086] Next, a second embodiment will be described. FIG. 2 is a diagram
illustrating an example of the composition of a radio communication
system 10. The radio communication system 10 includes a mobile terminal
apparatus (Mobile Station, hereinafter called "terminal") 100, radio base
station apparatuses (Evolved UTRAN NodeB (ENB), hereinafter called "base
station") 200a and 200b, and a serving gateway (hereinafter called
"gateway") 300.

[0087] The base stations 200a and 200b are radio communication apparatuses
which perform radio communication with the terminal 100. The base
stations 200a and 200b are connected by wire to the gateway 300, and are
able to transmit and receive data signals (hereinafter, called "data") to
and from the gateway 300 and the terminal 100. Furthermore, the base
stations 200a and 200b are also able to forward data between each other.
In the example in FIG. 1, two base stations 200a and 200b are depicted,
but there may be three or more base stations.

[0088] The terminal 100 is a radio communication apparatus, such as a
mobile telephone, portable information terminal apparatus, or the like,
which carries out radio communication by radio connection with the base
stations 200a and 200b. The terminal 100 is able to receive data
transmitted from the base stations 200a and 200b, by radio communication.
Furthermore, the terminal 100 is also able to transmit data to the base
stations 200a and 200b, by radio communication. In the present
specification, the direction from the base stations 200a and 200b to the
terminal 100 is a called a "down link" (DL) and the direction from the
terminal 100 to the base stations 200a and 200b is called an "up link"
(UL). In the example in FIG. 1, only one terminal 100 is depicted, but
there may also be a plurality of terminals 100 which have a radio
connection with the base station 200a, and one or a plurality of
terminals 100 which have a radio connection with the base station 200b.

[0089] In the example in FIG. 1, the two base stations 200a and 200b both
have the same composition, and are described as base station 200, unless
determined otherwise. The example in FIG. 1 depicts a situation where the
terminal 100 performs radio communication with the base station 200a in
the range of the cell of the base station 200a, and moves to the range of
the cell of the base station 200b, which is an adjacent base station.

[0090] <Examples of Composition of Base Station 200 and Terminal
100>

[0091] Next, respective examples of the composition of the base station
200 and the terminal 100 will be described. FIG. 3 is a diagram
illustrating the base station 200 relating to the second embodiment of
the invention, and FIG. 4 is a diagram illustrating an example of the
composition of the terminal 100.

[0092] The base station 200 includes a radio transmission and reception
unit 210, an RLC protocol control unit 220, a memory unit 230, a call
control unit 240, a facing E-NodeB IF unit (hereinafter, called "facing
ENB IF unit") 250, and a GW IF unit 260.

[0093] The radio transmission and reception unit 210 transmits a radio
signal to the terminal 100 and receives a radio signal transmitted from
the terminal 100. The radio transmission and reception unit 210, for
example, is able to read out data stored in the memory unit 230, convert
the data to a radio signal by applying error correction encoding
processing, modulation processing, frequency conversion processing, and
the like, to the data, and then transmit the radio signal to the terminal
100. Furthermore, upon receiving a radio signal from the terminal 100,
for example, the radio transmission and reception unit 210 is able to
extract data by applying frequency conversion process, demodulation
processing and error correction decoding processing, and the like, to the
radio signal, and then output this data to the RLC protocol control unit
220. Moreover, if the radio signal received from the terminal 100 is an
Ack signal (reception notification), then the radio transmission and
reception unit 210 is able to output the Ack signal to the RLC protocol
control unit 220. The Ack signal is, for example, a response signal when
data transmitted from a transmitter side, or the like, is received
correctly on a receiver side, and may also be called an affirmative
response or a confirmation response, or the like. For example, besides
being a response signal relating to data, the Ack signal may also be a
response signal relating to a control signal transmitted by the base
station 200 to the terminal 100.

[0094] Furthermore, the radio transmission and reception unit 210 includes
a radio wave condition notification unit 211. When the base station 200
is received a signal (or message) indicating "Measurement Reports"
transmitted from the terminal 100, for example, the radio wave condition
notification unit 211 extracts the radio quality between terminal 100 and
the base station 200. The radio wave information notification unit 211
reports the extracted radio quality to the handover decision unit 241.
Furthermore, the radio wave information notification unit 211 is also
able to hold the extracted radio quality in a radio wave condition table
233 which is stored in the memory unit 230. Alternatively, the radio wave
condition notification unit 211 is able to measure the radio quality of
each adjacent cell (or adjacent area), based on the radio signal of the
Ack signal, or the like, which is received from the terminal 100, and
hold the measured radio quality in the radio wave condition table 233 in
the memory unit 230. For example, the radio wave condition notification
unit 211 is able to hold the radio quality in terms of the electrical
power of a received radio signal, or the noise in relation to this power,
in the radio wave condition table 233. FIG. 15 illustrates an example of
a radio wave condition table 233, the details of which are described
hereinafter.

[0095] The RLC protocol control unit 220 stores the data output from the
radio transmission and reception unit 210, in the memory unit 230, judges
whether or not there is the data retransmission, based on an Ack signal
output from the radio transmission and reception unit 210, and implements
retransmission control if there is to be the retransmission. For example,
if an Ack signal is input from the radio transmission and reception unit
210 within a first threshold time period after the radio transmission and
reception unit 210 is transmitted data to the mobile terminal apparatus
100, then the RLC protocol control unit 220 decides not to perform the
data retransmission. On the other hand, if an Ack signal was not received
within a first threshold time period after the data is transmitted, then
the RLC protocol control unit 220 decides to perform the data
retransmission. Upon deciding to perform the data retransmission, the RLC
protocol control unit 220 reads out the retransmission data from the
memory unit 230 and outputs the data to the radio transmission and
reception unit 210, whereby the data is transmitted (or retransmitted) to
the terminal 100.

[0096] Moreover, the RLC protocol control unit 220 also includes a first
data communication condition gathering unit 221. The first data
communication condition gathering unit 221 saves the condition in which
the retransmission occurred, for each call, in a retransmission
information table 231 inside the memory unit 230. FIG. 5 illustrates an
example of a situation where the retransmission is occurred, and FIG. 6
illustrates an example of the retransmission information table 231.

[0097] In the example in FIG. 5, the serving base station (S-ENB) 200a
receives data of SDU-A to SDU-C from the gateway (GW) 300 (S10), and the
serving base station 200a transmits the respective data of SDU-A to SDU-C
to the terminal 100, in PDU units. An SDU is a unit of data which is, for
example, transmitted from the gateway 300 to the base station 200a or
transmitted between the base stations 200a and 200b. One or a plurality
of PDUs are included in a SDU, and the base station 200a, for example, is
able to transmit data to the terminal 100 in PDU units. In the present
specification, one SDU includes six PDUs, and SDU-A includes PDUs having
sequence numbers SN1 to SN6, SDU-B includes PDUs having sequence numbers
SN7 to SN13, and SDU-C includes PDUs having sequence numbers SN14 to
SN20. For example, since each SDU includes one or a plurality of PDUs,
then an SDU can be regarded as a data group. In the following
description, where appropriate, a base station which is the source of a
handover, to which a terminal 100 is connected, is called a serving base
station, and a base station which is the destination of the handover, is
called a target base station. When the terminal 100 is switched base
station connection by means of a handover, then the target base station
becomes the serving base station.

[0098] In the example in FIG. 5, the base station 200a transmits the PDUs
having sequence numbers SN1 to SN6, to the terminal 100 (S200, S220), and
receives Ack signals corresponding to the PDUs having sequence numbers
SN1 to SN3 (S210). Here, the base station 200a does not be able to detect
reception of Ack signals for the sequence numbers SN4 to SN6 within a
first threshold time period (S230), and is therefore retransmitted PDUs
having sequence numbers SN4 to SN6 (S240). The control of undetected Ack
signals and retransmitting of PDUs is implemented by the RLC protocol
control unit 220.

[0099]FIG. 6 is a diagram illustrating an example of the retransmission
information table 231 created by the first data communication condition
gathering unit 221, in a situation such as this. A flag indicating
whether or not a retransmission is occurred is stored for each call (or
each terminal 100), in the retransmission information table 231. In the
example in FIG. 6, an identification ID of the terminal 100 and a flag
indicating the presence or absence of the retransmission are stored; a
flag indicating that the retransmission does not be performed (for
example, "0") is stored in respect of the terminal UEID#1, and a flag
indicating that the retransmission is performed (for example, "1") is
stored in respect of the terminal UEID#2. It is also possible to store
the presence or absence of the retransmission within a monitoring period
in the retransmission information table 231. In this case, for example,
the first data communication condition gathering unit 221 is able to
store the presence or absence of the retransmission for each call in the
memory unit 230, read out the presence or absence of the retransmission
during a monitoring time period back in time from the handover decision,
and store this information in the retransmission information table 231.

[0100] Returning to FIG. 3, the memory unit 230 stores data received by
the radio transmission and reception unit 210 via the RLC protocol
control unit 220, and data forwarded respectively from an adjacent base
station or the gateway 300 via the GW IF unit 260 or the facing ENB IF
unit 250, and the like. The stored data is read out as and when
appropriate and transmitted from the radio transmission and reception
unit 210 to the terminal 100, or forwarded from the data forwarding
processing unit 244 to the target base station 200b which is the handover
destination. As described above, the memory unit 230 stores the
retransmission information table 231 (for example, FIG. 6), a statistical
information table 232 (for example, FIG. 13A), and a radio wave condition
table 233 (for example, FIG. 17A). The details of the statistical
information table 232 and the radio wave condition table 233 are
described hereinafter.

[0101] The call control unit 240 controls the transmission, reception and
forwarding of data, and the like, between the terminal 100 and the
gateway 300, and an adjacent base station 200. Furthermore, the call
control unit 240 is able to read out (or recover) data from the memory
unit 230, for example, and forward data to the adjacent base station 200b
via the facing ENB IF unit 250. The call control unit 240 includes a
handover decision unit 241, a second data communication condition
gathering unit 242, a forwarding data determination unit 243 and a data
forwarding processing unit 244.

[0102] The forwarding data determination unit 270 in the first embodiment
corresponds, for example, to the radio wave condition notification unit
211, the first data communication condition gathering unit 221, and the
second data communication condition gathering unit 242. Furthermore, the
data forwarding processing unit 280 in the first embodiment corresponds
to the data forwarding processing unit 244 and the facing ENB IF unit
250, for instance.

[0103] The handover decision unit 241 decides whether or not handover is
necessary, and the handover destination, and the like, based on the radio
quality reported by the radio wave condition notification unit 211. The
handover decision unit 241 decides that handover is to be performed, if
the reception power value measured by the terminal 100 as the radio
quality is equal to or less than a second threshold value. Furthermore,
the handover decision unit 241 determines the base station 200 having the
highest reception power value, of the reception power values in the other
base stations 200 measured by the terminal 100, as the handover
destination base station 200b. The handover decision unit 241 outputs the
identification information of the handover destination base station 200b,
and the like, to the forwarding data determination unit 243.

[0104] The second data communication condition gathering unit 242 gathers
a data communication condition for each cell belonging to an adjacent
base station 200 (hereinafter, called "adjacent cells"), and saves the
gathered data communication condition as statistical information in the
statistical information table 232 in the memory unit 230. FIG. 7 is a
diagram illustrating an example of a situation where a statistical
information table 232 is created. Similarly to the example in FIG. 5, the
base station 200 retransmits the PDUs having sequence numbers SN4 to SN6
(S200 to S240). Thereupon, the base station 200 receives "Measurement
Reports", decides to carry out handover (S250, S260), and stores the data
communication condition in the statistical information table 232, for
each adjacent cell according to the retransmission information table 231.
FIG. 13A illustrates an example of the statistical information table 232,
and the details thereof are described hereinafter. For instance, if the
retransmission is performed before a handover decision, then the second
data communication condition gathering unit 242 counts up the
"retransmission" items relating to the "cell" item of the handover
destination. On the other hand, if the retransmission does not be
performed before the handover decision, then the second data
communication condition gathering unit 242 counts up the "no
retransmission" items relating to that "cell" item.

[0105] Returning to FIG. 3, the forwarding data determination unit 243
determines the forwarding data that is to be forwarded to the handover
destination base station 200b, by means of any one of the retransmission
information table 231, the statistical information table 232 or the radio
wave condition table 233, or a combination of these tables 231 to 233.
The kind of data which is determined by the forwarding data determination
unit 243 as forwarding data is described hereinafter. The forwarding data
determination unit 243 outputs information relating to the determined
forwarding data, such as an SDU identification number, for example, to
the data forwarding processing unit 244.

[0106] The data forwarding processing unit 244 reads out the corresponding
forwarding data from the memory unit 230 based on information relating to
the forwarding data determined by the forwarding data determination unit
243, and outputs this forwarding data to the facing ENB IF unit 250.

[0107] The facing ENB IF unit 250 is an interface which is used when data,
and the like, is transmitted and received to and from an adjacent base
station 200b. The facing ENB IF unit 250 can, for example, convert the
forwarding data output from the data forwarding processing unit 244 into
a signal of a format that can be forwarded to the adjacent base station
200b (for example, an X2 format signal), and transmits the converted
signal. Moreover, the facing ENB IF unit 250 can also receive a signal of
this format transmitted from the adjacent base station 200b, extract
data, and the like, and output this data to the call control unit 240.

[0108] The GW IF unit 260 is an interface which is used when data, and the
like, is transmitted and received to and from the gateway 300. The GW IF
unit 260 can, for example, convert data stored in the memory unit 230
into a signal of a format that can be forwarded to the gateway 300 (for
example, the S1 format signal) and transmits this signal. Furthermore,
the GW IF unit 260 is able to receive a signal of this format transmitted
from the gateway 300, extract the data, and store the data in the memory
unit 230.

[0109] Next, an example of the composition of the terminal 100 will be
described. As illustrated in FIG. 4, for instance, the terminal 100
includes a radio transmission and reception unit 110, a call control unit
120, a RLC protocol control unit 130 and a memory 140.

[0110] The radio transmission and reception unit 110 is able to receive a
radio signal transmitted from the base station 200, and is also able to
transmit a radio signal to the base station 200. The radio transmission
and reception unit 110, for example, receives a radio signal transmitted
from the base station 200, applies frequency conversion processing,
demodulation processing, error correction decoding processing, and the
like, to the received radio signal, and extracts data, a control signal,
and the like, from the radio signal. Furthermore, the radio transmission
and reception unit 110 applies error correction encoding processing,
modulation processing, frequency conversion processing, and the like, to
the data, and the like, output from the call control unit 120, and
converts the data to a radio signal.

[0111] The call control unit 120 decides what kind of data to transmit to
the base station 200, and the like. The call control unit 120 can store
the data output from the radio transmission and reception unit 110, for
example, in the memory unit 140, and can also read out data to be
transmitted to the base station 200, from the memory unit 140, and output
this data to the radio transmission and reception unit 110.

[0112] The RLC protocol control unit 130 judges whether or not it is
possible to correctly decode the data or control signal received by the
radio transmission and reception unit 110, based on an error detection
code, such as a CRC (Cyclic Redundancy Check), which is appended to the
data, for example. For instance, if the RLC protocol control unit 130
judged that the data or signal is decoded correctly, then it generates an
Ack signal and instructs the radio transmission and reception unit 110 to
transmit this Ack signal to the base station 200. By this means, the
terminal 100 is able to transmit an Ack signal to the base station 200.
If the RLC protocol control unit 130 judges that the data or signal could
not be decoded correctly, then it does not perform any particular action.
In this case, it is possible to transmit a Nack signal, but from the
viewpoint of making efficient use of radio resources, for example, it is
supposed that the base station 200 does not transmit a Nack signal.
Incidentally, a Nack signal is a response signal which is transmitted in
cases where it does not be possible to receive data transmitted from the
transmitter side, correctly on the receiver side, for example, and this
signal may also be called a negative response, or the like.

[0113] The memory unit 140 is able to store data output from the call
control unit 120 or to store whether or not an Ack signal output from the
RLC protocol control unit 130 is transmitted. Data stored in the memory
unit 140 can be read out as appropriate from the call control unit 120,
or the like.

[0114] <Operational Examples>

[0115] Next, operational examples will be described. In these operational
examples, as illustrated in FIG. 2, for instance, a serving base station
200a which is a handover source base station determines handover of a
terminal 100, forwards data to a target base station 200b, which is a
handover destination base station, and transmits the data to the terminal
100. In this example, the serving base station 200a decides to perform
handover and the terminal 100 switches connection destination to the base
station 200b in a situation where data that ought to be transmitted to
the terminal 100 has not yet been transmitted. There are the following
four patterns in the operational example. More specifically:

1) When forwarding data is determined based on the retransmission status
which is held for each call; 2) When forwarding data is determined based
on a data communication condition, such as the retransmission occurrence
rate, which is held for each adjacent cell; 3) When forwarding data is
determined based on the radio wave condition between the handover source
base station 200a and the terminal 100; and 4) A combination of 1) to 3)
above. In 1) above, the serving base station 200a determines forwarding
data by using the retransmission information table 231, in 2) the serving
base station 200a determines forwarding data by using the statistical
information table 232, and in 3) the serving base station 200a determines
forwarding data by using the radio wave condition table 233. Moreover, in
4) the serving base station 200a determines the forwarding data based on
a combination of the retransmission information table 231, the
statistical information table 232 and the radio wave condition table 233.
Below, these four operational examples are described independently (as
first to fourth operational examples).

[0116] <First Operational Example>

[0117] The first operational example is an example of operation in a case
where forwarding data is determined based on the retransmission status
which is held by the serving base station 200a for each call. FIG. 8,
FIG. 9A and FIG. 10A and FIG. 10B respectively illustrate a sequence
diagram or a flowchart of the first operational example. Of these, FIG. 8
illustrates a sequence diagram of the first operational example. The
first operational example is now described with reference to FIG. 8.

[0118] Firstly, the serving base station (eNode-B) 200a receives data from
SDU-A to SDU-C, from the gateway 300 (S10). In this case, the serving
base station 200a stores the data of SDU-A to SDU-C in the memory unit
230, via the GW IF unit 260. Upon receiving an Ack signal in respect of a
PDU transmitted to the terminal 100, for example, the RLC protocol
control unit 220 of the serving base station 200a can delete the PDU
corresponding to the Ack signal from the memory unit 230.

[0119] The serving base station 200a which is received the data of SDU-A
to SDU-C transmits the data of SDU-A to the terminal (UE: User Equipment)
100, and sets the data of SDU-B and SDU-C to a state of awaiting
processing. More specifically, the serving base station 200a transmits,
to the terminal apparatus 100, PDUs having sequence numbers SN1 to SN6
which belong to SDU-A (S11, S13, S15 and S16), and receives Ack signals
for sequence numbers SN1 to SN3 from the terminal 100 (S12 and S14).
Furthermore, it is supposed that the serving base station 200a does not
receive Ack signals for sequence numbers SN4 to SN6 from the terminal
100.

[0120] In a state such as this, the serving base station 200a saves the
retransmission status of each call in the retransmission information
table 231 (see FIG. 6, for example) during a monitoring period
immediately before the handover decision (S18). As described above, for
example, the first data communication condition gathering unit 221 stores
information indicating whether or not the retransmission does not be
performed, for each terminal 100, in the retransmission information table
231. The indication of whether or not there is the retransmission is made
by means of the first data communication condition gathering unit 221
storing a "retransmission" flag in the retransmission information table
231, in respect of a call (or terminal 100) for which retransmission
control is performed, when the retransmission control is performed by the
RLC protocol control unit 220.

[0121] Next, the serving base station 200a decides to carry out handover
(S19). For example, the handover decision unit 241 decides to carry out
handover based on the radio quality included in a "Measurement Reports"
message.

[0122] Thereupon, the serving base station 200a performs recovery of data
forwarding (S20). For example, the call control unit 240 performs this
processing by reading out data from the RLC protocol control unit 220 via
the memory unit 230. The processing in this step S20 may be carried out
after the forwarding data is determined by the processing in step S21.

[0123] Next, the serving base station 200a determines the forwarding data
based on the retransmission status held for each call (S21). The
forwarding data is determined based on the retransmission information
table 231 by the forwarding data determination unit 243. FIG. 9A is a
flowchart illustrating an example of operation in a forwarding data
determination process according to the present operational example. This
process is carried out by the serving base station 200a when transferred
to the processing in S21.

[0124] Upon starting the forwarding data determination process (S210), the
forwarding data determination unit 243 judges the retransmission status
(S211). For example, in the retransmission information table 231 if a
retransmission information flag is on in respect of the terminal 100 in
question, then the forwarding data determination unit 243 determines that
the retransmission is performed for that terminal 100 and returns a
judgment of "retransmission". On the other hand, in the retransmission
information table 231 if the retransmission information flag is not on in
respect of the terminal 100 in question, then the forwarding data
determination unit 243 returns a judgment of "no retransmission".

[0125] FIG. 10A illustrates a sequence example in the case of the
"retransmission", and FIG. 10B illustrates a sequence example in the case
of "no retransmission". In the example in FIG. 10A, the serving base
station 200a was not able to confirm reception of an Ack signal in
relation to the PDUs having sequence numbers SN1 to SN3, during the first
threshold time period, and therefore is performed the retransmission in
respect of the PDUs having sequence numbers SN1 to SN3 (S30 to S32). In
this case, the retransmission information flag "1" is stored in the
retransmission information table 231, and the forwarding data
determination unit 243 returns the "retransmission" judgment.

[0126] On the other hand, in FIG. 10B, the serving base station 200a does
not be confirmed reception of an Ack signal in respect of sequence
numbers SN4 to SN6, but an actual retransmission does not be performed.
This is the same situation as steps S11 to S16 in FIG. 8. In this case,
the retransmission information flag in the retransmission information
table 231 is "0", and the forwarding data determination unit 243 judges
"no retransmission".

[0127] Returning to FIG. 9A, if the retransmission status is judged to be
"no retransmission" ("no retransmission" at S211), then the serving base
station 200a sets the "SDUs awaiting processing" which are scheduled to
be transmitted subsequently to the data under processing, as the
forwarding data (S212). In the case of "no retransmission", for example,
this indicates that the retransmission does not be performed in respect
of all of the PDUs in the SDU, and the SDUs which are "awaiting
processing" and for which processing for transmission to the terminal 200
has not yet been carried out may be forwarded to the target base station
200b. In the example in FIG. 10B, the retransmission does not be
performed in respect of any of the sequence numbers SN1 to SN6, and
therefore the forwarding data determination unit 243 sets "SDU-B" and the
"SDU-C" as forwarding data. If the retransmission does not be performed,
then the base station 200a determines that the data transmitted to the
terminal 100 was received correctly in the terminal 100, and the serving
base station 200a determines the data stored in the memory unit 230 which
is to be transmitted to the terminal 100, as the forwarding data.

[0128] On the other hand, if the retransmission status is judged to be
"retransmission" ("retransmission" at S211), then the serving base
station 200a sets SDUs which are under processing and SDUs which are
awaiting processing as the forwarding data (S213). For example, in the
example in FIG. 10A, the forwarding data determination unit 243 sets
"SDU-A", which is an SDU under processing and "SDU-B" and "SDU-C", which
are SDUs awaiting processing, as the forwarding data. If the
retransmission does not be performed, then the terminal 100 will not
necessarily receive all of the PDUs included in the SDU after the
retransmission. In cases such as these, the serving base station 200a
sets SDUs which are under transmission processing or under retransmission
processing, as forwarding data.

[0129] Returning to FIG. 8, when the serving base station 200a determines
the forwarding data (S21), the forwarding data is forwarded to the target
base station (eNodeB-b) 200b of the handover destination (S22). For
example, the forwarding data determination unit 243 reports information
about the determined forwarding data (for example, information
identifying the SDUs, such as SDU-A and SDU-B), to the data forwarding
processing unit 244, and the data forwarding processing unit 244 reads
out the forwarding data from the memory unit 230. The recovery of data
forwarding (S20) described above may be carried out by the processing in
S21, for example. In the example ("retransmission") in FIG. 10A, the
serving base station 200a reads out the data of "SDU-A", "SDU-B" and
"SDU-C" from the memory unit 230, and transmits this data to the target
base station 200b. Furthermore, in the example in FIG. 10B or FIG. 8
("not retransmission"), the serving base station 200a reads out the data
of "SDU-B" and "SDU-C" from the memory unit 230 and transmits this data
to the target base station 200b.

[0130] Thereupon, the terminal 100 switches connection destination to the
base station 200b, and the base station 200b which changes from a target
base station to a serving base station transmits the forwarding data to
the terminal 100 (S23). For instance, when the base station 200b receives
forwarding data from the handover source base station 200a, the base
station 200b stores the forwarding data in the memory unit 230 via the
facing ENB IF unit 250 and the call control unit 240. The radio
transmission and reception unit 210 reads out the forwarding data from
the memory unit 230 and transmits this data to the terminal 100 by radio
communication. In the case of the examples in FIG. 10A and FIG. 10B, for
instance, if the data of SDU-A to SDU-C is forwarded as the
"retransmission", then the base station 200b sequentially transmits data
from the sequence number SN1 of SDU-A, to the terminal 100, by radio
communication. On the other hand, if the data of SDU-B and SDU-C is
forwarded as "no retransmission", for instance, then the base station
200b sequentially transmits data from sequence number SN7 of SDU-B, to
the terminal 100, by radio communication.

[0131] In this way, in the first operational example, when the
retransmission is performed, the data of sequence number SN1 belonging to
SDU-A onwards, for example, is forwarded to the target base station 200b,
and therefore, after handover, the terminal 100 is able to receive the
data of sequence number SN1 onwards, from the base station 200b.
Consequently, if the retransmission is performed, then the data that was
the object of the retransmission is also forwarded, and therefore no data
loss occurs, even if a handover is performed since the terminal 100 can
receive the data subjected to the object of the retransmission from the
handover base station 200b. Moreover, when the retransmission does not be
performed, then data from the sequence number SN7 awaiting processing
onwards is forwarded and transmitted to the terminal 100, and therefore
the sequence numbers SN4 to SN6, for example, are not transmitted again
from the base station 200b at the handover destination. Consequently, the
PDUs having sequence numbers SN4 to SN6 are not transmitted in a
duplicated fashion from the base station 200b, and the terminal 100 does
not receive these PDUs in a duplicated fashion from the base station
200b.

[0132] In the first operational example, when the serving base station
200a determines forwarding data and forwarded the data (S22), the
sequence number of a PDU may also be reported. FIG. 11 is a diagram
illustrating an example of a sequence when a sequence number is reported.
The reported PDU sequence number (S24) represents, for example, the
sequence number of the PDU from which the handover destination base
station 200b starts transmission.

[0133] In this case, the first data communication condition gathering unit
221 stores the presence or absence of the retransmission, and the
sequence number of PDUs which are the object of an Ack signal, in the
retransmission information table 231. The forwarding data determination
unit 243 reads out the presence or absence of the retransmission, and the
sequence number of the PDU which is the object of the last Ack signal to
be received before the handover decision, from the retransmission
information table 231, and sets the sequence number following this
sequence number as the sequence number to be reported.

[0134] For example, in the example in FIG. 10A (the "retransmission"
example), the sequence number of the PDU for which the last Ack signal
was received before the handover decision is SN0, and therefore the
sequence number SN1 which follows this is set as the sequence number to
be reported.

[0135] On the other hand, in the case of "no retransmission", SDUs
awaiting processing are set as the forwarding data, and therefore the SDU
which is to be transmitted to the terminal 100 first among the SDUs
awaiting processing is set as the object SDU and the first sequence
number belonging to this object SDU may be set as the sequence number to
be reported. For example, in the example in FIG. 10B, SDU-B is the SDU
which is transmitted first to the terminal 100, from among the forwarding
data, and therefore the sequence number SN7 of the first PDU belonging to
SDU-B may be set as the sequence number to be reported. The sequence of
each PDU belonging to each of the SDUs is determined in advance and
stored in the memory unit 230, or the like, and therefore the forwarding
data determination unit 243 is able to use this information to determine
the sequence numbers.

[0136] In this way, since the serving base station 200a reports the
sequence number to the target base station 200b, then the handover
destination base station 200b is able to transmit PDUs sequentially from
the sequence number, to the terminal 100. Therefore, the base station
200b and the terminal 100 are able to further prevent duplicated
transmission, duplicated reception and data loss, by confirming the
sequence number, and so on.

[0137] <Second Operational Example>

[0138] Next, a second operational example is described, which is an
example of operation in a case where forwarding data is determined based
on a data communication condition, such as the retransmission occurrence
rate for each adjacent cell. In the second operational example, the
statistical information table 232 is used. FIG. 9B, and FIG. 12 to FIG.
14 are diagrams illustrating examples, such as sequence examples based on
the operational examples. Of these, FIG. 12 illustrates a sequence
example of the second operational example, and hence the second
operational example will be described with reference to FIG. 12.

[0139] As illustrated in FIG. 12, processing in relation to the data
transmission status is similar to the first operational example described
above. More specifically, the serving base station 200a transmits PDUs
having sequence numbers SN1 to SN6, to the terminal 100, and of these,
receives Ack signals in relation to the PDUs having sequence numbers SN1
to SN3. Moreover, the serving base station 200a is in a state where
sequence numbers SN4 to SN6 are not resent (S10 to S16). Moreover, the
serving base station 200a holds the retransmission status for each call
in the retransmission information table 231 (S18).

[0140] Furthermore, similarly to the first operational example, the
serving base station 200a decides to perform handover in respect of the
terminal 100 (S19) and carries out data recovery (S20).

[0141] Moreover, the serving base station 200a stores the retransmission
status and the retransmission occurrence rate for each adjacent cell in
the statistical information table 232 (S30). This processing is carried
out by the second data communication condition gathering unit 242, for
example.

[0142]FIG. 13A is a diagram illustrating an example of a statistical
information table 232. The statistical information table 232 includes the
respective items of "RLC procedure status" ("retransmission" and "no
retransmission"), "retransmission occurrence rate" and "quality judgment
result", for each adjacent area (or cell, hereinafter called "adjacent
cell").

[0143] In relation to the adjacent cells, a cell of an adjacent base
station to which it is possible to transfer by handover from the target
base station 200a is stored as an "adjacent cell" in the statistical
information table 232. FIG. 14 is a diagram illustrating an example of
the relationship of adjacent cells to a cell X of the serving base
station 200a. As expressed in FIG. 14, the adjacent cells relating to
cell X are cells A-1, B-1, C-1, D-1, E-1 and F-1.

[0144] A numerical value corresponding to the presence or absence of the
retransmission is stored in the "RLC procedure status" item, when there
is a handover to the respective target base stations of the adjacent
cells A-1 to F-1, for instance. For example, the second data
communication condition gathering unit 242 refers to the retransmission
information table 231 stored in the memory unit 230 and confirms whether
or not the flag indicating the retransmission is set to on in respect of
the terminal which is being handed over. If the retransmission flag is
on, then the second data communication condition gathering unit 242
counts up the numerical values stored in the "retransmission" item of the
"RLC procedure status", and stores the value of this count. Furthermore,
if the retransmission flag is off, then the second data communication
condition gathering unit 242 counts up the numerical values stored in the
"no retransmission" item and stores the value of this count. In the
example in FIG. 13A, when the terminal 100 is handed over from the
serving base station 200a in cell X to the base station in cell A-1, then
the number of times that the retransmission is performed to the base
station 200a within the monitoring time period is "0" times, and the
number of times that no retransmission is performed is "500" times. For
example, the terminal 100 is situated in the vicinity of the boundary
between cell X and cell A-1, and the retransmission status when
communicating with the base station 200a in cell X is stored in the
statistical information table 232.

[0145] The "retransmission occurrence rate" item stores the ratio of times
that the retransmission is performed in radio communication carried out
with the handover source base station, when handover is made to the base
station of the adjacent cell, based on the "RLC procedure status", for
example. In this calculation, the "retransmission occurrence rate" is a
numerical value which expresses the ratio of the number of
"retransmission" times with respect to the sum total of the respective
counts of "retransmission" and "no retransmission". In the example in
FIG. 13A, the number of "retransmission" times for the adjacent cell A-1
is "0", the sum total of the number of "retransmission" and "no
retransmission" times for the adjacent cell A-1 is "100", and therefore
the retransmission occurrence rate is "0%". The number of
"retransmission" times for the adjacent cell B-1 is "250", the sum total
is "500", and therefore the retransmission occurrence rate is "50%". The
"retransmission occurrence rate" is calculated by reading out the values
of the respective items which is stored as the "RLC procedure status" by
the second data communication condition gathering unit 242, for example,
and the calculation result is stored in the statistical information table
232 as the "retransmission occurrence rate" item.

[0146] A radio quality corresponding to the adjacent cells A-1 to F-1
which is judged based on the retransmission occurrence rate, for example,
is stored in the "quality judgment" item. In the example in FIG. 13B, if
the "retransmission occurrence rate" is 0% to 20%, then the radio quality
is judged to be "good", and if the "retransmission occurrence rate" is
20% to 100%, then the radio quality is judged to be "poor"; a "good" or
"poor" radio quality is stored accordingly in the "quality judgment" item
of the statistical information table 232. Thus, a third threshold value
is provided in relation to the "retransmission occurrence rate", and if
the "retransmission occurrence rate" is equal to or less than the third
threshold value, then the radio quality is judged to be "good" and if the
"retransmission occurrence rate" is greater than the third threshold
value, then the radio quality is judged to be "poor". In the example in
FIG. 13B, the third threshold value is set to "20%". In the example in
FIG. 13A and FIG. 14, for instance, the radio quality when it is decided
to hand over the terminal 100 to the base station in cell A-1 and when
the terminal 100 is situated in the range of the cell X in the vicinity
of the boundary between cell X and cell A-1 is judged to be "good",
because the retransmission occurrence rate is equal to or less than the
third threshold value. Furthermore, the radio quality when it is decided
to hand over the terminal 100 to the base station in cell B-1 and when
the terminal 100 is situated in the range of cell X in the vicinity of
the boundary between cell X and cell B-1 is judged to be "poor", because
the retransmission occurrence rate is greater than the third threshold
value. In FIG. 13B, the third threshold value is set to "20%", but it may
be a different value, taking account of various conditions, standards,
and the like. For example, the second data communication condition
gathering unit 242 reads out the value stored for the "retransmission
occurrence rate" in the statistical information table 232, compares this
value with the third threshold value, and stores either a "good" or a
"poor" radio quality in the "quality judgment" item, depending on which
value is larger.

[0147] In this way, the statistical information table 232 stores the
number of presence or absence of retransmission performed until the
current time, for each adjacent cell, and includes statistical
information relating to the presence and absence of the retransmission
for each adjacent cell; the radio quality is judged based on this
statistical information.

[0148] Returning to FIG. 12, the serving base station 200a then determines
forwarding data based on the radio quality status (S31). The forwarding
data determination process for determining the forwarding data is carried
out by means of the flowchart illustrated in FIG. 9B, for instance. For
example, the forwarding data determination unit 243 determines the
forwarding data by using the statistical information table 232 stored in
the memory unit 230.

[0149] When the forwarding data determination process starts (S300), the
forwarding data determination unit 243 judges the radio quality of the
handover destination cell (S301). This judgment is carried out by means
of the forwarding data determination unit 243 reading out the value
("good" or "poor") stored for the "quality judgment" item in the
statistical information table 232.

[0150] If the radio quality of the handover destination is "good" ("good
in S301), then the forwarding data determination unit 243 sets the "SDUs
awaiting processing" as the forwarding data (S302). For instance, in the
example in FIG. 12, when the serving base station 200a decides on
handover to a base station in the adjacent cell A-1, then the forwarding
data determination unit 243 judges that the "quality judgment" of the
adjacent cell A-1 is "good", based on the statistical information table
232. The forwarding data determination unit 243 determines the respective
data of "SDU-B" and "SDU-C" which are awaiting processing, as the
forwarding data.

[0151] In this case, the serving base station 200a does not receive an Ack
signal corresponding to the PDUs having sequence numbers SN4 to SN6.
However, the serving base station 200a judges that the radio quality when
moving from cell X to cell A-1 is "good". In a case such as this, the
forwarding data determination unit 243 judges that the PDUs having
sequence numbers SN4 to SN6 is received correctly by the terminal 100,
and sets "SDU-B" which starts from the sequence number SN7 and "SDU-C",
as the forwarding data.

[0152] Returning to FIG. 9B, on the other hand, if the forwarding data
determination unit 243 judges that the radio quality of the handover
destination is "poor" ("poor" at S301), then the "SDUs under processing
and SDUs awaiting processing" are set as forwarding data (S303). For
instance, in the example in FIG. 12, if the serving base station 200a
decides a base station having cell B-1 as the handover destination (S19),
then the forwarding data determination unit 243 judges the "quality
judgment" of the adjacent cell B-1 to be "poor", based on the statistical
information table 232. The forwarding data determination unit 243
determines the respective data of "SDU-A" which is under processing and
"SDU-B" and "SDU-C" which are awaiting processing, as the forwarding
data.

[0153] In this case, the serving base station 200a does not receive an Ack
signal corresponding to the PDUs having sequence numbers SN4 to SN6.
Furthermore, the serving base station 200a judges the radio quality when
moving from cell X to cell B to be "poor". In a case such as this, the
forwarding data determination unit 243 judges that it does not be
possible to receive the PDUs having sequence numbers SN4 to SN6 correctly
in the terminal 100, and sets the SDUs from "SDU-A" onwards which include
sequence numbers SN4 to SN6, as the forwarding data.

[0154] Returning to FIG. 12, the serving base station 200a transmits the
determined forwarding data to the handover destination base station 200b
(S22), and the handover destination base station 200b transmits the
forwarding data to the terminal 100 (S23). For instance, if the radio
quality is "good" ("good" at S301 in FIG. 9B), then the serving base
station 200a forwards the respective data of "SDU-B" and "SDU-C" which
are awaiting processing, to the target base station 200b. The base
station 200b then transmits the data sequentially to the terminal 100, by
radio communication, from the PDU having sequence number SN7. On the
other hand, if the radio quality is "poor" ("poor" at step S301 in FIG.
9B), then the serving base station 200a transmits "SDU-A" which is under
processing, and "SDU-B" and "SDU-C" which are awaiting processing, to the
target base station 200b. The base station 200b then transmits the data
sequentially to the terminal 100, by radio communication, from the PDU
having sequence number SN1 (S23).

[0155] In this way, in the present operational example, the serving base
station 200a calculates the retransmission occurrence rate based on
statistical information relating to the presence or absence of the
retransmission for each adjacent cell, and judges the radio quality for
each adjacent cell based on the retransmission occurrence rate. If the
radio quality is judged to be "good", then the serving base station 200a
judges that the data under processing could be received correctly in the
terminal 100, and the data awaiting processing is set as forwarding data.
Consequently, the base station 200b of the handover destination does not
transmit the data under processing in a duplicated fashion, and the
terminal 100 does not receive the data under processing in a duplicated
fashion from the handover destination base station 200b.

[0156] On the other hand, if the radio quality is judged to be "poor",
then the serving base station 200a judges that the data under processing
could not be received correctly in the terminal 100, and the data under
processing and the data awaiting processing are set as the forwarding
data. Consequently, since the data under processing in the handover
source base station 200a is transmitted to the terminal 100 by the
handover destination base station 200b, then there is no loss of data in
the terminal 100.

[0157] Similarly to the first operational example described above, the
serving base station 200a may report the sequence number of the PDU which
represents the start of transmission, to the target base station 200b.
FIG. 15 is a sequence diagram illustrating an operational example in a
case where a sequence number is reported. In this case also, similarly to
the first operational example, the first data communication condition
gathering unit 221, for instance, stores the presence or absence of
retransmission in the retransmission information table 231 and stores the
sequence numbers of PDUs that are the object of a received Ack signal.
The forwarding data determination unit 243 reads out the sequence number
of the PDU that was the object of the last Ack signal received before the
handover decision, from the retransmission information table 231, sets
the sequence number following this sequence number as the sequence number
to be reported, and then reports this number (S24). By reporting the
sequence number, the handover destination base station 200b is able to
transmit the PDU having the reported sequence number to the terminal 100,
and therefore it is possible to further prevent duplicated transmission,
duplicated reception and data loss.

[0158] <Third Operational Example>

[0159] Next, a third operational example, which is an example of a case
where forwarding data is determined based on the radio wave condition,
will be described. FIG. 16 to FIG. 19 and FIG. 9B illustrates examples of
sequence diagrams, and the like, according to this operational example.

[0160]FIG. 16 is a diagram illustrating a sequence example according to
the third operational example. In this third operational example,
similarly to the first and second operational examples, the serving base
station 200a receives Ack signals in relation to the PDUs having sequence
numbers SN1 to SN3, and does not perform the retransmission of the PDUs
having sequence numbers SN4 to SN6 (S10 to S16).

[0161] In the third operational example, the serving base station 200a
measures the radio quality between the serving base station 200a and the
terminal 100, based on the signal received from the terminal 100, and
stores the measured radio quality in the radio wave condition table 233
as a radio wave condition (S35). For example, the radio wave condition
notification unit 211 measures the reception power of the Ack signal
received from the terminal 100, and the noise in relation to this
reception power, and the like, as the radio quality, and stores this in
the radio wave condition table 233. Alternatively, upon receiving a
"Measurement Reports" message including a radio quality measured at the
terminal 100, the radio wave condition notification unit 211 extracts the
radio quality included in the message and stores this information as the
radio wave condition in the radio wave condition table 233.

[0162]FIG. 17A is a diagram illustrating an example of a radio wave
condition table 233. The radio wave condition table 233 includes the
items "radio wave condition" and "quality judgment", for each adjacent
cell.

[0163] If the terminal 100 is located in cell X of the serving base
station 200a, as illustrated in FIG. 18, for example, then the adjacent
cells are the cells A-1 to F-1 of the base stations to which the terminal
can be handed over from the serving base station 200a.

[0164] The radio quality measured or extracted by the serving base station
200a is stored in the "radio wave condition" item. For example, in the
example in FIG. 17A, the information stored as the "radio wave condition"
for cell "A-1" is the radio quality between the terminal 100 and the
serving base station 200a when the terminal 100 is located in the cell X
in the vicinity of the boundary between the cell X and the cell A-1.
Alternatively, the radio wave condition notification unit 211 extracts
the radio quality and cell ID included in the received "Measurement
Reports" message, and stores the extracted radio quality in the
corresponding "radio wave condition" item.

[0165] The "quality judgment" item is stored as "good" when the measured
radio quality is equal to or higher than a fourth threshold value, and is
stored as "poor" when the measured radio quality is lower than the fourth
threshold value, respectively for each adjacent cell. In FIG. 17B, the
fourth threshold value is set to "2 dB", and the radio wave condition for
the adjacent cell B-1 is "1.5 dB", which is smaller than "2 dB", and
therefore a quality judgment of "poor" is stored, whereas the radio wave
condition for the adjacent cell A-1 is "2.5 dB", which is larger than "2
dB", and therefore a quality judgment of "good" is stored.

[0166] Returning to FIG. 16, in this way, the radio quality is stored in
the radio wave condition table 233 of the serving base station 200a
(S35). In this case, similarly to the first operational example, for
instance, it is possible to store the radio quality which is measured or
extracted during a monitoring time period before the handover decision.
For example, the radio wave condition notification unit 211 stores the
measured or extracted radio quality in the memory unit 230, and then
reads out the radio quality corresponding to the monitoring period before
the handover decision (S19), from the memory unit 230, and stores this
radio quality in the radio wave condition table 233.

[0167] Thereupon, the serving base station 200a decides to carry out
handover (S19), performs data recovery (S20), and then determines the
forwarding data (S40). The forwarding data determination process can be
implemented in accordance with the flowchart illustrated in FIG. 9B, for
example, similarly to the second operational example described above.

[0168] When the forwarding data determination process starts (S300), the
forwarding data determination unit 243 judges the radio quality (S301).
For example, the forwarding data determination unit 243 judges the radio
quality based on whether "good" or "poor" is stored in the "quality
judgment" item corresponding to the "adjacent cell" belonging to the base
station which is the handover destination, in the radio wave condition
table 233 (S301).

[0169] If the radio quality of the handover destination is "good" ("good"
at step S301), then the forwarding data determination unit 243 sets "SDUs
awaiting processing" as the forwarding data (S302). In this case,
similarly to the second operational example described above, provided
that the radio quality is "good", then it is probable that the terminal
100 will be able to correctly receive data under processing which is
transmitted, even if the serving base station 200a does not confirm
reception of an Ack signal, and in this case, "SDUs awaiting processing"
are set as the forwarding data. In the example in FIG. 16 and FIG. 17A,
the serving base station 200a does not receive Ack signals in respect of
the sequence numbers SN4 to SN6, when the terminal 100 is handed over to
a base station having the adjacent cell "A-1". However, since the radio
quality of the adjacent cell "A-1" is "good", then the serving base
station 200a sets "SDU-B" which starts from sequence number SN7, and
"SDU-C" which follows "SDU-B", as the forwarding data.

[0170] On the other hand, if the radio quality of the handover destination
is "poor" ("poor" at S302), then the forwarding data determination unit
243 sets the "SDU under processing and SDUs awaiting processing" as the
forwarding data (S303). In this case, similarly to the second operational
example described above, if the radio quality is "poor", then the
possibility that the data under processing was received correctly even if
the serving base station 200a does not confirm reception of an Ack
signal, is low compared to a case where the radio quality is "good". In
this case, the forwarding data is set to include the "SDU under
processing", which includes data for which reception of an Ack signal
does not be confirmed, and the "SDUs awaiting processing" which follow
the SDU under processing. In the example in FIG. 16 and FIG. 17A, when
the terminal 100 is handed over to the base station having adjacent cell
"B-1", the radio quality of the adjacent cell "B-1" is "poor", and
therefore the serving base station 200a sets the "SDU-A under processing"
and the "SDUs awaiting processing" as the forwarding data.

[0171] Upon having determined the forwarding data (S40), the serving base
station 200a transmits the data to the target base station 200b which is
the handover destination (S22). The serving base station 200a forwards
the "SDUs awaiting processing" when the radio quality is "good", and
forwards the "SDU under processing and SDUs awaiting processing" when the
radio quality is "poor".

[0172] The handover destination base station 200b transmits the forwarded
data to the terminal 100 (S23).

[0173] In this way, in the third operational example, the serving base
station 200a judges the radio quality for each adjacent cell based on the
radio wave condition of each adjacent cell. Similarly to the second
operational example, if the radio quality is judged to be "good", the
base station 200a judges that the data under processing could be received
correctly in the terminal 100 and the data awaiting processing is set as
the forwarding data. Consequently, the handover destination base station
200b does not transmit the data under processing in a duplicated fashion,
and the terminal 100 does not receive the data under processing which
already is received, in a duplicated fashion, from the handover
destination base station 200b.

[0174] Furthermore, if the radio quality is judged to be "poor", then the
serving base station 200a judges that the data under processing could not
be received correctly by the terminal 100, and sets the data under
processing and the data awaiting processing as the forwarding data.
Consequently, the handover destination base station 200b is able to
transmit the data which is under processing in the handover source base
station 200a, to the terminal 100, and therefore no data loss occurs in
the terminal 100.

[0175] In the third operational example also, similarly to the first and
second operational examples, the serving base station 200a may report the
sequence number of the PDU which represents the start of transmission.
FIG. 19 is a sequence diagram illustrating an operational example in a
case where the sequence number is reported. In this case also, similarly
to the first and second operational examples, the first data
communication condition gathering unit 221 stores the presence or absence
of the retransmission, and the sequence numbers of PDUs which are the
object of received Ack signals, in the retransmission information table
231. The forwarding data determination unit 243 then reads out the
sequence number of the PDU which is the object of the last Ack signal
received before the handover decision, from the retransmission
information table 231, sets the sequence number following this sequence
number as the sequence number to be reported, and then reports this
sequence number (S24). By reporting the sequence number, the handover
destination base station 200b can transmit the PDU of the reported
sequence number, to the terminal 100, and it is possible to further
prevent duplicated transmission, duplicated reception and data loss.
These examples will be explained consecutively in the following.

[0176] <Fourth Operational Example>

[0177] Next, a fourth operational example, in other words, a combination
of the first to third operational examples will be described. The fourth
operational example is a method which determines forwarding data by means
of a combination of the retransmission status for each call (first
operational example), and the radio quality status (second or third
operational example). In the case of a combination of this kind, there
are two possible operational examples. The first is one where forwarding
data is determined by a combination of the first operational example (the
retransmission status of each call) and the second operational example
(the radio quality status based on retransmission occurrence rate). The
second is one where forwarding data is determined by a combination of the
first operational example (the retransmission status of each call) and
the third operational example (the radio quality status based on the
radio wave condition).

[0178] <1. Combination of First Operational Example and Second
Operational Example>

[0179] Firstly, an example which combines the first and second operational
examples will be described. FIG. 20 to FIG. 24 are diagrams illustrating
sequence diagrams or flowcharts of this operational example, and the
like. Parts which perform the same processing as the first and second
operational examples are labeled with the same reference numerals.

[0180] Of these, FIG. 20 is a sequence diagram of the present operational
example. This operational example is now described with reference to FIG.
20. In the present operational example, similarly to the first to third
operational examples, the serving base station 200a transmits PDUs having
sequence numbers SN1 to SN6 which are included in the SDU-A, to the
terminal 100, and receives Ack signals corresponding to the PDUs having
sequence numbers SN1 to SN3, from the terminal 100 (S10 to S16).

[0181] Similarly to the first operational example, the serving base
station 200a holds the retransmission status for each call, in the
retransmission information table 231 (S18). For example, as illustrated
in FIG. 6, the first data communication condition gathering unit 221
stores the presence or absence of the retransmission for each terminal
100, in the retransmission information table 231.

[0182] Thereupon, the serving base station 200a carries out the
determination of handover execution and data recovery (S19 and S20).

[0183] Thereupon, similarly to the second operational example, the serving
base station 200a stores the retransmission status and the retransmission
occurrence rate for each adjacent cell, in the statistical information
table 232 (S30). For instance, the second data communication condition
gathering unit 242 stores the number of retransmission and the number of
no retransmission in the "retransmission" or "no retransmission" items of
the statistical information table 232, based on the retransmission status
of each call, as illustrated in FIG. 13A and FIG. 13B. The second data
communication condition gathering unit 242 calculates the retransmission
occurrence rate from the stored number of times and stores the calculated
rate in the "retransmission occurrence rate" item. Moreover, the second
data communication condition gathering unit 242 compares the
retransmission occurrence rate with the third threshold value and stores
"good" or "poor" indicating the radio quality, as a "quality judgment" in
the statistical information table 232.

[0184] Next, the serving base station 200a determines the forwarding data
based on the retransmission status of each call and the radio quality
status (S50). For example, the forwarding data determination unit 243
determines forwarding data based on the retransmission information table
231 and the statistical information table 232 stored in the memory unit
230.

[0185]FIG. 21 is a flowchart illustrating an example of a forwarding data
determination process according to the present operational example. When
processing is started (S400), similarly to the first operational example
(S211), the forwarding data determination unit 243 judges the presence or
absence of the retransmission to the terminal 100 for which it is decided
to perform handover, based on the retransmission information table 231
(S401). For instance, the forwarding data determination unit 243 judges
"retransmission" if the "retransmission or no retransmission" item in the
retransmission information table 231 is on ("1"), and judges "no
retransmission" if the "retransmission or no retransmission" item is off
("0").

[0186] If the retransmission does not be performed ("no retransmission" at
step S401), then the forwarding data determination unit 243 judges the
radio quality of the handover destination cell (S402). Similarly to the
second operational example (S301), the forwarding data determination unit
243 judges the radio quality based on the statistical information table
232 stored in step S30. More specifically, the forwarding data
determination unit 243 judges the radio quality to be "good" if "good" is
stored as the "quality judgment" in the statistical information table
232, and judges the radio quality to be "poor" if "poor" is stored as the
"quality judgment" in the statistical information table 232.

[0187] If the radio quality of the handover destination cell is "good"
("good" in S402), then the forwarding data determination unit 243 sets
the "SDUs awaiting processing" which are scheduled to be transmitted
after the data under processing, as the forwarding data (S403). In this
case, if the retransmission does not be performed in respect of the data
under processing ("no retransmission" at S401) and the radio quality of
the handover destination is "good" ("good" at S402), then it can be
judged that the terminal 100 will probably be able to correctly receive
the data under processing, even if an Ack signal does not be received
from the terminal 100, for example. In cases such as this, the serving
base station 200a determines the "SDUs awaiting processing" which are
scheduled to be transmitted to the terminal 100 after the data under
processing, as the forwarding data.

[0188] On the other hand, if the radio quality of the handover destination
cell is "poor" ("poor" at S402), then the forwarding data determination
unit 243 determines the data under processing ("SDU under processing")
and the "SDUs awaiting processing" which are scheduled to be transmitted
after the data under processing, as the forwarding data (S404). In this
case, if the radio quality of the handover destination is "poor", then
the possibility that the data under processing is received correctly in
the terminal 100 even though an Ack signal does not be received from the
terminal 100 is low compared to a case were the radio quality is "good".
In cases such as this, the serving base station 200a determines the data
under processing ("SDU under processing") and the "SDUs awaiting
processing" as the forwarding data.

[0189] Furthermore, when the retransmission is performed ("retransmission"
at S401), the forwarding data determination unit 243 determines the "SDU
under processing" and the "SDUs awaiting processing" as the forwarding
data (S404). This is because in conditions where the retransmission is
performed, there is a high possibility that the retransmission will be
performed again compared to conditions where the retransmission is not
performed, even if the serving base station 200a does not receive an Ack
signal in respect of the retransmitted data. Therefore, if the
retransmission is performed, then the "SDU under processing" which is
transmitted is set as forwarding data, regardless of whether or not an
Ack signal is received in respect of the retransmitted data.

[0190]FIG. 22 is a diagram illustrating an example of the kind of data
that is set as forwarding data in conditions where PDUs having sequence
numbers SN1 to SN6 is transmitted to the terminal 100, as in FIG. 20, and
where an Ack signal is transmitted in respect of the PDUs having sequence
numbers SN1 to SN3. If there is no retransmission and the radio quality
is "good", then the "SDU-B" and "SDU-C", which are "SDUs awaiting
processing" are set as the forwarding data. In other cases, "SDU-A" to
"SDU-C" are set as the forwarding data.

[0191] Returning to FIG. 20, the serving base station 200a forwards the
determined forwarding data to the base station 200b of the handover
destination (S22), and the base station 200b of the handover destination
transmits the forwarded data to the terminal 100 (S23).

[0192] FIG. 27 is a diagram illustrating an example of a sequence when a
handover is performed to a base station of an adjacent cell which has
"good" radio quality, without the retransmission being performed, under
the conditions in FIG. 20. In this example, the forwarding data is the
data from sequence number SN7 onwards, and the PDUs from sequence number
SN7 onwards are transmitted from the handover destination base station
200b.

[0193] In this operational example also, similarly to the first to third
operational examples, the serving base station 200a may transmit a
sequence number to the target base station 200b. FIG. 23 is a diagram
illustrating a sequence example including the reporting of a sequence
number. In this example also, the first data communication condition
gathering unit 221 stores sequence numbers of PDUs which are the object
of a received Ack signal, in the retransmission information table 231,
for instance, and the forwarding data determination unit 243 determines
the sequence number to report based on the determined forwarding data and
the stored sequence numbers. For example, in a situation such as that in
FIG. 23, when the forwarding data determination unit 243 determines the
"SDU under processing and SDUs awaiting processing" as the forwarding
data, the sequence number "SN4" may be reported. Furthermore, if the
forwarding data determination unit 243 determines the "SDUs awaiting
processing" as the forwarding data, then the sequence number "SN7" may be
reported.

[0194]FIG. 24 is a diagram illustrating an example of sequence numbers
which are reported in situations of this kind. If the retransmission
status is "no retransmission", and the radio quality is "good", then the
reported sequence number is SN"7", which his the first sequence number of
the PDU belonging to SDU-B which is "awaiting processing". If the
retransmission status is "no retransmission" and the radio quality is
"poor", then the reported sequence number is SN"4", which is the first
sequence number for which an Ack signal does not be received. If the
retransmission status is "retransmission", then the sequence number SN"4"
is reported, regardless of the radio quality.

[0195] In the present operational example, consequently, similarly to the
first and second operational examples, if the radio quality is judged to
be "good", then the base station 200a judges that the data under
processing could be received correctly in the terminal 100 and sets the
data awaiting processing as the forwarding data. Therefore, the base
station 200b of the handover destination does not transmit the data under
processing in a duplicated fashion, and the terminal 100 does not receive
the data under processing in a duplicated fashion from the handover
destination base station 200b.

[0196] Furthermore, if the retransmission is performed, or if the
retransmission does not be performed and the radio quality is "poor",
then the serving base station 200a judges that the data under processing
could not be received correctly in the terminal 100, and sets the data
under processing and the data awaiting processing as the forwarding data.
Consequently, since the data under processing in the handover source base
station 200a is transmitted to the terminal 100 by the handover
destination base station 200b, then there is no loss of data in the
terminal 100.

[0197] <2. Combination of First Operational Example and Third
Operational Example>

[0198] Next, a fourth operational example, in other words, a combination
of the first operational example and the third operational example will
be described. FIG. 25 is a diagram illustrating a sequence example
according to this operational example and FIG. 26 is a diagram
illustrating a sequence example in a case where a sequence number is
reported.

[0199] In this operational example, similarly to the first operational
example, the serving base station 200a holds the retransmission status
for each call during a monitoring period, for example, in the
retransmission information table 231 (see FIG. 6, for example) (S18).
Furthermore, similarly to the third operational example, the serving base
station 200a holds the radio wave condition during the monitoring period,
for example, in the radio wave condition table 233 (see FIG. 17A, for
example) (S35).

[0200] The serving base station 200a then decides handover (S19), performs
data recovery (S20), and determines the forwarding data based on the
retransmission information table 231 and the radio wave condition table
233 (S60). The forwarding data determination process is, for example,
similar to that described in the combination of the first and second
operational examples which was explained above. More specifically, as
illustrated in FIG. 21, if no retransmission is performed to the terminal
100 being handed over ("no retransmission" at S401) and if the radio
quality of the handover destination cell is "good" ("good" at S402), then
the forwarding data determination unit 243 sets as the "SDUs awaiting
processing" as the forwarding data (S403). On the other hand, if
retransmission is performed ("retransmission" at S401) or retransmission
is not performed but the radio quality is "poor" ("poor" at S402), then
the forwarding data determination unit 243 sets the "SDU under processing
and the SDUs awaiting processing" as the forwarding data (S404). The fact
that the forwarding data is determined by using the radio wave condition
table 233 differs from the combination of the first and second
operational examples described above. A sequence example relating to a
case where a sequence number is reported is illustrated in FIG. 26, but
the kind of sequence number reported is similar to the combination of the
first and second operational examples described above.

[0201] In the present operational example, consequently, similarly to the
first and third operational examples, if the radio quality is judged to
be "good", the base station 200a judges that the data under processing
could be received correctly in the terminal 100 and sets the data
awaiting processing as the forwarding data. Therefore, the base station
200b of the handover destination does not transmit the data under
processing in a duplicated fashion, and the terminal 100 does not receive
the data under processing in a duplicated fashion from the handover
destination base station 200b.

[0202] Furthermore, if the retransmission does not be performed but the
radio quality is "poor", or if the retransmission is performed, then the
serving base station 200a judges that the data under processing could not
be received correctly in the terminal 100, and sets the data under
processing and the data awaiting processing as the forwarding data.
Consequently, since the data under processing in the handover source base
station 200a is transmitted to the terminal 100 by the handover
destination base station 200b, then there is no loss of data in the
terminal 100.

Third Embodiment

[0203] Next, a third embodiment of the invention will be described. FIG.
28 is a diagram illustrating a further example of the composition of a
base station 200 and a terminal 100.

[0204] The base station 200 includes an antenna 271, a DSP (Digital Signal
Processing unit) 272, a CPU 273, a ROM (Read Only Memory) 274, a RAM
(Random Access Memory) 275, and a memory unit 230.

[0205] For example, the functions of the call control unit 240 of the base
station 200 (see FIG. 3, for example) in the second embodiment can be
achieved by coordinated operation of the CPU 273, the ROM 274 and the RAM
275. Moreover, for example, the functions of the facing ENB IF unit 250
in the second embodiment can be achieved by causing the DSP 272 to
operate by transmitting an instruction to the DSP 272 from the CPU 273.
Moreover, the functions of the radio transmission and reception unit 210
in the second embodiment can be achieved by operation of the DSP 272 and
the antenna 271, for example. The functions of the RLC protocol control
unit 220 according to the second embodiment can be achieved by
coordinated operation of the CPU 273, the ROM 274 and the RAM 275, or by
operation of the DSP 272.

[0206] On the other hand, the terminal 100 includes an antenna 171, a DSP
172, a CPU 173, a ROM 174, a RAM 175 and a memory unit 140.

[0207] For example, the functions of the call control unit 120 of the
terminal 100 (see FIG. 4, for example) in the second embodiment are
achieved by coordinated operation of the CPU 173, the ROM 174 and the RAM
175. Moreover, for example, the functions of the radio transmission and
reception unit 110 in the second embodiment can be achieved by causing
the DSP 172 and the antenna 171 to operate by transmitting an instruction
to the DSP 172 from the CPU 173. The functions of the RLC protocol
control unit 130 according to the second embodiment can be achieved by
coordinated operation of the CPU 173, the ROM 174 and the RAM 175, or by
operation of the DSP 172.

[0208] Consequently, in the base station 200 and the terminal 100
illustrated in FIG. 28, it is possible to achieve the respective first to
fourth operational examples described above, similarly to the second
embodiment.

Fourth Embodiment

[0209] Next, a fourth embodiment of the invention will be described. The
fourth operational example of the second embodiment was described by way
of examples of a combination of the first operational example and the
second operational example (for example, FIG. 20 to FIG. 24) and a
combination of the first operational example and the third operational
example (for example, FIG. 25 and FIG. 26). In this fourth embodiment, an
operational example using a combination of the second operational example
and the third operational example is described. FIG. 31 illustrates a
sequence example of the fourth embodiment, and FIG. 32 illustrates an
example of SDUs which are forwarded, in the fourth embodiment.

[0210] The respective compositional examples of the base stations 200a and
200b, and the terminal 100 are similar to those of the second embodiment
(see FIG. 3 and FIG. 4, for example). Furthermore, the conditions under
which the terminal 100 performs handover from the serving base station
200a to the target base station 200b are also similar to the second
embodiment.

[0211] Furthermore, in FIG. 31, processes which are the same as those of
the second embodiment are labeled with the same numbers. As illustrated
in FIG. 31, the serving base station 200a receives SDU-A to SDU-C as data
from the gateway 300, and transmits PDUs having sequence numbers SN1 to
SN6 which are contained in SDU-A, to the terminal 100. Of these PDUs, the
serving base station 200a receives an Ack signal corresponding to the
PDUs having sequence numbers SN1 to SN3, and does not receive an Ack
signal corresponding to the PDUs having sequence numbers SN4 to SN6 (S11
to S16).

[0212] In communication conditions of this kind, the serving base station
200a holds the retransmission status for each call, in the retransmission
information table 231 (S18), similarly to the second operational example
of the second embodiment. For instance, the first data communication
condition gathering unit 221 stores the presence or absence of the
retransmission in the retransmission information table 231.

[0213] Furthermore, the serving base station 200a stores the radio quality
in the radio wave condition table 233, similarly to the third operational
example of the second embodiment (S35). For instance, the radio wave
condition notification unit 211 stores the radio quality included in a
Measurement Report received from the terminal 100, or a radio quality
measured based on a radio signal received from the terminal 100, in the
radio wave condition table 233. For example, the radio wave condition
notification unit 211 stores values in the "radio wave condition" and
"quality judgment" items of the radio wave condition table 233 (see FIG.
17A, for example).

[0214] Next, the serving base station 200a decides to carry out handover
(S19), and recovers data forwarding (S20). The serving base station 200a
then stores the retransmission status and the retransmission occurrence
rate for each adjacent cell in the statistical information table 232
(S30). For example, the second data communication condition gathering
unit 242 stores values, or the like, in the respective items, "RLC
procedure status" (the count value of "retransmission" or "no
retransmission"), "retransmission occurrence rate" and "quality
judgment", of the statistical information table 232 (for example, FIG.
13A), based on the retransmission information table 231.

[0215] Thereupon, the serving base station 200a judges the forwarding data
based on the radio quality status (S65). For example, the forwarding data
determination unit 243 judges forwarding data based on the statistical
information table 232 and the radio wave condition table 233 stored in
the memory unit 230. In this case, the forwarding data determination unit
243 judges the forwarding data based on the "quality judgment" in the
statistical information table 232 and the "quality judgment" in the radio
wave status table 233.

[0216]FIG. 32 illustrates an example of judging the "radio quality" based
on this combination. For example, if the "quality judgment" in the
statistical information table 232 (the "retransmission occurrence rate"
in FIG. 32) is "good", and the "quality judgment" in the radio wave
condition table 233 (the "radio wave condition" in FIG. 32) is "good",
then the forwarding data determination unit 243 judges that the radio
quality is "good". In the case of combinations other than this, the
forwarding data determination unit 243 judges the radio quality to be
"poor".

[0217] The forwarding data is determined by applying this judgment result
of the radio quality to the "radio quality of the handover destination
cell" (S301) in the forwarding data determination process (see FIG. 9B,
for example), similarly to the second embodiment.

[0218] For example, under communication conditions such as those
illustrated in FIG. 31, if the radio quality is "good" ("good" at S301),
then SDU-B and SDU-C which are awaiting processing are set as forwarding
data, and if the radio quality is "poor" ("poor" at S301), then SDU-A
which is under processing and SDU-B and SDU-C which are awaiting
processing are set as forwarding data (see FIG. 32, for example).

[0219] In this way, if the radio quality is good, the serving base station
200a judges that the data transmitted to the terminal 100 (for example,
the PDUs having sequence numbers SN4 to SN6) is received in the terminal
100, even if an Ack signal does not be received in respect of that data.
In situations such as this, the serving base station 200a sets the "SDUs
awaiting processing" (for example, the SDU-B onwards) as forwarding data.

[0220] Consequently, the serving base station 200a does not transmit the
"SDU under processing" to the target base station 200b, and hence the
target base station 200b does not transmit the data under processing to
the terminal 100 in a duplicated fashion, and the terminal 100 does not
receive the data under processing in a duplicated fashion.

[0221] On the other hand, the serving base station 200a sets the "SDU
under processing" and the "SDUs awaiting processing" as the forwarding
data, if an Ack signal does not be received within a threshold time
period in respect of the data transmitted to the terminal 100 (if the
"retransmission occurrence rate" is "poor") or if the radio quality is
less than a threshold value (if the "radio wave condition" is "poor").

[0222] Consequently, the serving base station 200a transmits the data
under processing to the terminal 100, and therefore the terminal 100 is
also able to receive data which could not be received correctly, and data
loss does not occur.

[0223] As illustrated in FIG. 32, for example, in the fourth embodiment,
the overall "quality judgment" is judged to be "good", if the "quality
judgment" is "good" in both the statistical information table 232 and the
radio wave condition table 233. Consequently, the reliability when the
overall "quality judgment" is "good" can be raised compared to the case
of the second operational example or the third operational example of the
second embodiment.

[0224] In FIG. 31, the processing from the determination of forwarding
data (S65) onwards involves the same processing as the second embodiment
(S22, S23). In this case, the serving base station 200a may report the
sequence number of the PDU from which the target base station 200b starts
transmission (S24).

Fifth Embodiment

[0225] Next, a fifth embodiment of the invention will be described. The
second to fourth embodiments were described with reference to an example
where a serving base station 200a decides handover after all of the PDUs
included in an SDU is transmitted. In the example relating to the fifth
embodiment, the serving base station 200a makes a handover decision
before transmitting all of the PDUs contained in an SDU, and hence there
are PDUs awaiting transmission (or PDUs which does not yet be
transmitted).

[0226]FIG. 33 illustrates a sequence example according to the fifth
embodiment, and although the details thereof are described below, the
following communication conditions can be envisaged, for instance. More
specifically, the serving base station 200a receives SDU-A to SDU-C from
the gateway as data addressed to the terminal 100. The serving base
station 200a transmits the PDUs having sequence numbers SN1 to SN4 which
are included in SDU-A, to the terminal 100 (S11 to S73), and then decides
to carry out handover (S19). In this case, the PDUs having sequence
numbers SN5 to SN6 included in SDU-A are awaiting transmission (or does
not yet be transmitted). The compositional examples of the SDU and the
PDU are the same as those of the second embodiment, for instance.

[0227] In this case, if the serving base station 200a determines that
transmission is possible in the transmission possible/not possible
judgment step (S76), then the serving base station 200a transmits the
sequence numbers SN5 to SN6 which are awaiting transmission, to the
terminal 100 (S77). In this situation, the serving base station 200a
forwards SDU-B and SDU-C which have sequence numbers from SN7 onwards, to
the target base station 200b, as forwarding data (S22).

[0228] In this way, if there is data awaiting transmission (for example,
data which has not yet been transmitted), and the serving base station
200a according to the fifth embodiment is capable of transmitting this
data, then the serving base station 200a transmits the data awaiting
transmission to the terminal 100 and does not forward the data awaiting
transmission to the target base station 200b.

[0229] By this means, data awaiting transmission (for example, PDUs having
sequence numbers SN5 to SN6) is not transmitted to the terminal 100 from
the target base station 200b, and the target base station 200b does not
transmit the data awaiting transmission to the terminal 100 in a
duplicated fashion, as well as the serving base station 200a.
Furthermore, in this case, the terminal 100 does not receive the data
awaiting transmission (for example, sequence numbers SN5 to SN6) from the
two base stations 200a and 200b, and hence there is no duplicated
reception.

[0230] Moreover, since the data awaiting transmission (for example,
sequence numbers SN5 to SN6) is transmitted from the serving base station
200a (for example, in step S77), then it is possible to avoid situations
where the data awaiting transmission is not transmitted and a data loss
occurs.

[0231] This is described in detail below. FIG. 33 to FIG. 44 are diagrams
illustrating operational examples according to the fifth embodiment, and
the like. The compositional examples of the radio communication system
10, the serving base station 200a, the target base station 200b, and the
terminal 100 are the same as those of the second to fourth embodiments
(for example, see FIG. 2 to FIG. 4, etc.)

[0232] Furthermore, the compositional examples of the SDUs and the PDUs,
and the like, are also similar to the second to fourth embodiments; for
instance, the SDU-A includes PDUs having sequence numbers SN1 to SN6.
Moreover, SDU-B includes PDUs having sequence numbers SN7 to SN12, and
SDU-C includes PDUs having sequence numbers SN13 to SN18.

[0233] The operational example according to the fifth embodiment includes
the following four patterns, similarly to the second embodiment.

[0234] 1) When forwarding data is determined based on the retransmission
status which is held for each call;

[0235] 2) When forwarding data is determined based on a data communication
condition, such as the retransmission occurrence rate, which is held for
each adjacent cell;

[0236] 3) When forwarding data is determined based on the radio wave
condition between the serving base station 200a and the terminal 100; and

[0237] 4) A combination of 1) to 3) above.

[0238] Below, four operational examples (first to fourth operational
examples) are described, similarly to the second embodiment.

[0239] <First Operational Example>

[0240] In the first operational example of the fifth embodiment, a
handover decision is made when there is data awaiting transmission, and
furthermore the forwarding data is determined based on the retransmission
status. FIG. 33 illustrates a sequence example of the first operational
example; FIG. 34 illustrates an example of the retransmission information
table 231 in the first operational example; and FIG. 35 illustrates an
example of transmission possible/not possible judgment processing in the
first operational example. Furthermore, FIG. 36A and FIG. 36B
respectively illustrate a sequence example in the first operational
example, and FIG. 37 illustrates an example of a forwarding data
determination process according to the first operational example. In FIG.
33 and other drawings, processes which are the same as the first
operational example of the second embodiment, and the like, are labeled
with the same reference numerals.

[0241] As described above, conditions of the following kinds can be
envisaged as the communication condition. More specifically, the serving
base station 200a receives the data from SDU-A to SDU-C, from the gateway
300, and transmits the data from sequence number SN1 to SN4 which is
included in SDU-A, to the terminal 100 (S11 to S73). Of these, the
serving base station 200a receives from the terminal 100 an Ack signal in
respect of the PDUs having sequence numbers SN1 and SN2, and does not
receive an Ack signal in respect of the PDUs having sequence numbers SN3
and SN4. The serving base station 200a decides to perform handover of the
terminal 100 (S19), and the PDUs having sequence numbers SN5 and SN6 are
awaiting transmission.

[0242] In communication conditions of this kind, the serving base station
200a stores the retransmission status and transmission time of each call
in the retransmission information table 231 (S75). For example, the first
data communication condition gathering unit 221 detects the presence or
absence of a retransmission for each call (for instance, for each
terminal 100), and also detects the transmission time of each call.

[0243] For instance, the first data communication condition gathering unit
221 is able to detect the transmission time by measuring the transmission
interval between the PDUs transmitted from the radio transmission and
reception unit 210. In the example in FIG. 33, the first data
communication condition gathering unit 221 measures the transmission time
by measuring the time from the transmission of the PDU having sequence
number SN1 until the transmission of the PDU having sequence number SN2.
The transmission time can be found by the first data communication
condition gathering unit 221 by, for instance, measuring an average time
from the transmission times of a plurality of PDUs or by finding the
longest or shortest time taken to transmit one PDU.

[0244]FIG. 34 is a diagram illustrating a configuration example of the
retransmission information table 231 according to the fifth embodiment.
The example of the retransmission information table 231 illustrated in
FIG. 34 has an "average time" item for the transmission time, and the
first data communication condition gathering unit 221 stores a value in
this "average time".

[0245] Returning to FIG. 33, after deciding to carry out handover (S19),
the serving base station 200a decides whether or not transmission is
possible (S76). The serving base station 200a judges whether or not it is
possible to transmit the PDUs having sequence numbers SN5 and SN6, which
are awaiting transmission, by carrying out a transmission possible/not
possible judgment process, for example.

[0246]FIG. 35 is a flowchart illustrating an example of a transmission
possible/not possible judgment process. The transmission possible/not
possible judgment process is carried out by the forwarding data
determination unit 243 or the handover decision unit 241, for instance.

[0247] Upon starting the transmission possible/not possible judgment
process (S760), the serving base station 200a judges whether or not the
predicted transmission time is longer then the maximum reservable time
(S761). Here, the predicted transmission time and the maximum reservable
time will be described.

[0248]FIG. 36A and FIG. 36B are diagrams for respectively describing the
predicted transmission time and the maximum reservable time.

[0249] The predicted transmission time is a time period based on the
number of PDUs awaiting transmission, for instance, and is the time taken
to complete transmission of the last PDU awaiting transmission after the
handover decision (S19). For example, in the examples in FIG. 36A and
FIG. 36B, the predicted transmission time is the time from the handover
decision (S19) until the end of transmission of the PDU having sequence
number SN6, which is the last PDU awaiting transmission (*1).

[0250] On the other hand, the maximum reservable time is, for example, the
time from the handover decision (S19) until handover is established
(S78). Establishment of handover (S78) means a state immediately before
reporting a handover request to the handover destination base station
200b, when a handover destination is determined by a handover decision.
Consequently, the maximum reservable time is, for example, the time from
the handover decision (S19) until immediately before transmitting a
handover request (S78) (*2).

[0251] The predicted transmission time may also be a time period which
changes in accordance with the number of PDUs awaiting transmission or
the transmission time taken to transmit one PDU. For example, the
forwarding data determination unit 243 is able to calculate the predicted
transmission time by reading out the "average time" in the retransmission
information table 231 (for example, FIG. 34) and multiplying by the
number of PDUs awaiting transmission.

[0252] On the other hand, the maximum reservable time is a process which
is carried out within a prescribed time period, for example, from the
handover decision until the transmitting of a handover request. For
example, the maximum reservable time is stored in the memory unit 230 and
the forwarding data determination unit 243 is able to read out the
maximum reservable time stored in the memory unit 230.

[0253] As illustrated in FIG. 36A, when the predicted transmission time
(*1) is less than the maximum reservable time (*2), then in this case,
there is sufficient time to transmit the sequence numbers SN5 and SN6
awaiting transmission to the terminal 100.

[0254] On the other hand, if the predicted transmission time (*1) is equal
to or greater than the maximum reservable time (*2), as illustrated in
FIG. 36B, then the maximum reservable time (*2) would be exceeded if the
PDU having sequence number SN6 which is awaiting transmission are
transmitted, and therefore transmission within the maximum reservable
time period (*2) is not possible.

[0255] Consequently, as illustrated in FIG. 35, if the predicted
transmission time is less than the maximum reservable time (Yes at S761),
then the forwarding data determination unit 243 can decide to transmit
the PDUs awaiting transmission, since there is sufficient time to be able
to transmit the PDUs awaiting transmission (S762).

[0256] On the other hand, if the predicted transmission time is equal to
or greater than the maximum reservable time (No at S761), then there is
not sufficient time to transmit the PDUs awaiting transmission within the
maximum reservable time, and therefore the forwarding data determination
unit 243 can decide not to transmit the PDUs awaiting transmission
(S764). For example, it is possible to avoid the occurrence of data loss
due to the handover source base station and the terminal 100 becoming
unable to communicate before the terminal 100 receives the untransmitted
PDUs.

[0257] When the forwarding data determination unit 243 determines whether
or not transmission is possible (S762 or S764), the transmission possible
or not possible judgment process is terminated (S763).

[0258] By means of the foregoing, the transmission possible or not
possible judgment is made (S76) and it is determined whether or not
transmission of the PDUs awaiting transmission is possible, for example.

[0259] Returning to FIG. 33, upon carrying out the transmission possible
or not possible judgment process (S76), the serving base station 200a
either transmits the data awaiting transmission or does not transmit the
data awaiting transmission, to the terminal 100, in accordance with this
judgment. The example in FIG. 33 is one where the PDUs having sequence
numbers SN5 and SN6 which are awaiting transmission are transmitted
(S77).

[0260] Next, the serving base station 200a establishes handover (S78) and
transmits a handover request to the target base station 200b (S79). For
example, the handover decision unit 241 generates a handover request to
the handover destination base station 200b (or the target base station
200b), in accordance with the handover decision (S19), and is able to
transmit this request to the target base station 200b via the facing
E-Node IF 250.

[0261] The serving base station 200a then performs recovery of data
forwarding (S20) and determines the forwarding data (S80).

[0262]FIG. 37 is a flowchart illustrating an operational example of a
forwarding data determination process. The serving base station 200a is
able to determine the forwarding data by carrying out a forwarding data
determination process.

[0264] For example, the forwarding data determination unit 243 determines
that the retransmission is performed to the terminal 100 and returns a
"retransmission" judgment, when the "retransmission or no retransmission"
item is on for the terminal 100 in question, in the retransmission
information table 231 (see FIG. 34, for example). On the other hand, the
forwarding data determination unit 243 returns a "no retransmission"
judgment when the "retransmission or no retransmission" item is off for
the terminal 100 in question.

[0265] The forwarding data determination unit 243 sets the "SDU under
processing" and the "SDUs awaiting processing" as forwarding data, if the
judgment is "retransmission" for the terminal 100 which is being handed
over ("retransmission" at S801) (S804).

[0266] For example, there are also cases where the retransmission is
performed again when the serving base station 200a is performed the
retransmission in respect of the PDUs having sequence numbers SN3 and SN4
in the example of communication conditions illustrated in FIG. 33. In a
case of this kind, SDU-A which includes the PDUs having sequence numbers
SN3 and SN4, and also SDU-B and SDU-C which are awaiting processing, are
set as forwarding data.

[0267] On the other hand, if the retransmission status is judged to be "no
retransmission" ("no retransmission" at S801), then the forwarding data
determination unit 243 judges whether or not the predicted transmission
time is less than the maximum reservable time (S802). This judgment
involves the same processing as S761 in the transmission possible/not
possible judgment process (S76), for example, judging whether or not
there is sufficient time to be able to transmit all of the PDUs awaiting
transmission (see FIG. 36A, for instance), or judging whether or not the
PDUs awaiting transmission is all transmitted by the judgment process in
S761.

[0268] Consequently, the forwarding data determination unit 243 judges
that there is sufficient time to be able to transmit all of the PDUs
awaiting transmission, or judges that all of the PDUs awaiting
transmission is transmitted, and sets the "SDUs awaiting processing" as
the forwarding data (S803), if the predicted transmission time is less
than the maximum reservable time (Yes at S802).

[0269] For example, the PDUs having sequence numbers SN5 and SN6 which are
awaiting transmission are transmitted to the terminal 100 (S77) in
accordance with the transmission possible or not possible process (S76),
and if the predicted transmission time is less than the maximum
reservable time (Yes at S802), then SDU-B and SDU-C are set as the
forwarding data.

[0270] On the other hand, if the predicted transmission time is equal to
or greater than the maximum reservable time (No at S802), then the
forwarding data determination unit 243 sets the "SDUs under processing"
and the "SDUs awaiting processing" as the forwarding data (S804). For
example, if the predicted transmission time is equal to or greater than
the maximum reservable time, the PDUs awaiting transmission are not
transmitted to the terminal 100 (see FIG. 36B, for example).
Consequently, in order to prevent data loss, for example, the SDU-A which
includes PDUs having sequence numbers SN5 and SN6, and the SDU-B and
SDU-C which are awaiting processing, are set as forwarding data.

[0271] The serving base station 200a determines the forwarding data by the
forwarding data determination process (S80) described above. Returning to
FIG. 33, the serving base station 200a transmits the forwarding data to
the target base station 200b in accordance with the decision made in the
forwarding data determination process (S22).

[0272] In this case, similarly to the second embodiment, the serving base
station 200a may report the sequence number of the PDU at which the
target base station 200b starts transmission to the terminal 100 (S24).
For example, in the example in FIG. 33, the serving base station 200a can
report the sequence number SN7 (if Yes at S802) or the sequence number
SN5 (if No at S802). Furthermore, if there is the retransmission in
respect of the sequence numbers SN3 and SN4 in the example in FIG. 33,
for instance (Yes at S801), then the serving base station 200a can report
the sequence number SN3. By reporting a sequence number, it is possible
to further prevent duplicated transmission, duplicated reception, and
data loss, similarly to the first operational example in the second
embodiment.

[0273] Thereupon, the serving base station 200a transmits resource
allocation information (DL allocation) for the downlink direction (the
direction from the base stations 200a and 200b to the terminal 100), to
the terminal 100 (S90). This allocation information may include, for
instance, identification information for the target base station 200b
which is the handover destination of the terminal 100.

[0274] The terminal 100 carries out synchronization processing with the
target base station 200b (S91), and the target base station 200b becomes
the serving base station and is able to receive forwarding data (S23). In
this case, if a sequence number is reported (S24), for example, then the
terminal 100 is able to receive the PDUs from the reported sequence
number onwards, and if a sequence number is not reported, then the
terminal 100 is able to receive the PDUs from the PDU having the first
sequence number, of the PDUs included in the SDU.

[0275] <Second Operational Example>

[0276] Next, a second operational example will be described. The second
operational example is an example where, for example, a handover decision
is made when there is data awaiting transmission, and the forwarding data
is determined based on the retransmission occurrence rate, and the like.
FIG. 38 illustrates a sequence example relating to the second operational
example. Furthermore, FIG. 39 is a flowchart illustrating an operational
example of a forwarding data determination process according to the
second operational example.

[0277] As illustrated in FIG. 38, in respect of the communication
conditions, similarly to the first operational example, the serving base
station 200a transmits PDUs having sequence numbers SN1 to SN4 to the
terminal 100, and of these, receives Ack signals corresponding to the
PDUs having sequence numbers SN1 and SN2 (S11 to S73). Furthermore, the
serving base station 200a makes a handover decision before transmitting
the sequence numbers SN5 and SN6 (S19), and therefore the PDUs having
sequence numbers SN5 and SN6 are awaiting transmission.

[0278] In communication conditions of this kind, similarly to the first
operational example, the serving base station 200a holds the
retransmission status for each call, and the transmission time, in the
retransmission information table 231 (S75). For instance, the first data
communication condition gathering unit 221 stores the presence or absence
of the retransmission for each call (for each terminal 100, for
instance), and the transmission time, during the monitoring period before
a handover decision, in the retransmission information table 231 (see
FIG. 34, for example).

[0279] Thereupon, the serving base station 200a decides to carry out
handover (S19) and stores the retransmission status and the
retransmission occurrence rate for each adjacent cell in the statistical
information table 232 (S30).

[0280] Similarly to the second embodiment, for example, the second data
communication condition gathering unit 242 writes the count value of the
"RLC procedure status" (either "retransmission" or "no retransmission")
in the statistical information table 232 (see FIG. 13A, for example),
based on the retransmission information table 231 (see FIG. 34, for
example). The second data communication condition gathering unit 242
calculates the retransmission occurrence rate based on the count value,
and stores the calculated value as the "retransmission occurrence rate"
item in the statistical information table 232. The second data
communication condition gathering unit 242 makes a quality judgment based
on the "retransmission occurrence rate" in the statistical information
table 232, and stores either "good" or "poor" as the "quality judgment"
item in the statistical information table 232.

[0281] Thereupon, the serving base station 200a carries out transmission
possible/not possible judgment (S76). The transmission possible/not
possible judgment process is the same as that of the first operational
example according to the fifth embodiment (see FIG. 35, for example). For
instance, the forwarding data determination unit 243 decides that the
PDUs awaiting transmission can be transmitted to the terminal 100 if the
predicted transmission time is less than the maximum reservable time (Yes
at S761 in FIG. 35). On the other hand, the forwarding data determination
unit 243 decides not to transmit the PDUs awaiting transmission (S764),
if the predicted transmission time is equal to or greater than the
maximum reservable time (No at S761).

[0282] Returning to FIG. 38, the serving base station 200a transmits the
PDUs awaiting transmission to the terminal 100 (S77), if it is judged
that transmission is possible in the transmission possible/not possible
judgment.

[0283] Next, the serving base station 200a establishes handover (S78) and
transmits a handover request for the terminal 100, to the target base
station 200b (S79). The target base station 200b is able to transmit a
response to the handover request, to the serving base station 200a.

[0284] The serving base station 200a then performs recovery of data
forwarding (S20) and determines the forwarding data based on the radio
quality status (S81).

[0285]FIG. 39 is a flowchart illustrating an operational example of a
forwarding data determination process according to the second operational
example. For example, the forwarding data determination unit 243
determines the forwarding data by using the statistical information table
232 stored in the memory unit 230.

[0286] Upon starting the forwarding data determination process (S810), the
forwarding data determination unit 243 judges the radio quality of the
handover destination cell (S811). The forwarding data determination unit
243 reads out and assesses the "quality judgment" relating to the target
base station 200b in the statistical information table 232.

[0287] The forwarding data determination unit 243 sets the "SDU under
processing" and the "SDUs awaiting processing" as the forwarding data
(S814), if the "quality judgment" for the target base station 200b is
"poor" ("poor" at step S811).

[0288] For example, since no Ack signal is received from the terminal 100
in respect of the PDUs having sequence numbers SN3 and SN4, and the radio
quality is "poor", then the possibility that the PDUs having these
sequence numbers is correctly received in the terminal 100 is lower than
in a case where the radio quality is "good". Therefore, in a case of this
kind, the forwarding data determination unit 243 sets the "SDU under
processing" and the "SDUs awaiting processing" as the forwarding data.

[0289] On the other hand, if the "quality judgment" for the target base
station 200b is "good" ("good" at S811), then the forwarding data
determination unit 243 judges whether or not the predicted transmission
time is less than the maximum reservable time (S812).

[0290] Similarly to the judgment in the first operational example (S802 in
FIG. 37, for example), this judgment involves determining whether or not
it is possible to transmit the PDUs awaiting transmission, for example,
(or whether or not the PDUs awaiting transmission is transmitted). In the
second operational example, if it is judged that the PDUs awaiting
transmission can be transmitted, in the transmission possible or not
possible judgment processing (S76), then the PDUs awaiting transmission
are transmitted (S77), and if the PDUs awaiting transmission is
transmitted, then transmission of these PDUs to the target base station
200b can be omitted.

[0291] Consequently, if the predicted transmission time is less than the
maximum reservable time (Yes at S812), then the forwarding data
determination unit 243 sets the "SDUs awaiting processing" as the
forwarding data (S813). For example, if the PDUs having sequence numbers
SN5 and SN6 which are awaiting transmission can be transmitted (or is
transmitted), then the forwarding data determination unit 243 can
determine SDU-B and SDU-C, which are "SDUs awaiting processing", as the
forwarding data.

[0292] On the other hand, if the predicted transmission time is equal to
or greater than the maximum reservable time (No at S812), then the
forwarding data determination unit 243 sets the "SDUs under processing"
and the "SDUs awaiting processing" as the forwarding data (S814). For
example, if the PDUs having sequence numbers SN5 and SN6 which are
awaiting processing cannot be transmitted (or is transmitted), then the
forwarding data determination unit 243 can determine SDU-A, which is an
"SDU under processing", and SDU-B and SDU-C, which are "SDUs awaiting
processing", as the forwarding data. By means of the foregoing, the
serving base station 200a is able to determine the forwarding data.

[0293] Returning to FIG. 38, the serving base station 200a forwards the
determined forwarding data to the target base station 200b (S22). In this
case, similarly to the first operational example, the serving base
station 200a may report the sequence number of the PDU from which the
target base station 200b starts transmission (S24).

[0294] The serving base station 200a then transmits the downlink
allocation information (DL allocation) to the terminal 100 (S90), and the
terminal 100 carries out synchronization processing with the target base
station 200b, which is the handover destination (S91). The terminal 100
is able to receive the forwarded data from the base station 200b which
was the serving base station (S23).

[0295] <Third Operational Example>

[0296] Next, a third operational example of the fifth embodiment will be
described. The third operational example is an example where, for
example, a handover decision is made when there is data awaiting
transmission, and the forwarding data is determined based on the radio
wave condition, and the like. For example, FIG. 39 and FIG. 40 illustrate
a sequence example, or the like, according to this third operational
example.

[0297] FIG. 40 is a diagram illustrating a sequence example according to
the third operational example. In this third operational example, the
serving base station 200a transmits PDUs having sequence numbers SN1 to
SN4, to the terminal 100, and of these, receives Ack signals in relation
to the PDUs having sequence numbers SN1 and SN2 (S11 to S73).
Furthermore, the serving base station 200a makes a handover decision
before transmitting the sequence numbers SN5 and SN6 (S19), and therefore
the PDUs having sequence numbers SN5 and SN6 are awaiting transmission.

[0298] In communication conditions of this kind, the serving base station
200a holds the radio wave condition for a prescribed period of time
(S35). Similarly to the third operational example in the second
embodiment, the radio wave status notification unit 211 measures the
reception power of an Ack signal received from the terminal 100, or the
noise in relation to the reception power, or the like, as the radio
quality, and stores this as the radio wave condition in the radio wave
condition table 233. FIG. 17A is a diagram illustrating an example of a
radio wave condition table 233. Similarly to the third operational
example in the second embodiment, the radio wave condition notification
unit 211 stores a radio quality value in the item "radio wave condition"
of the radio wave condition table 233, and stores "good" or "poor" in the
"quality judgment" item by means of threshold value judgment, or the like
(see FIG. 17B, for example). The value stored as the "radio wave
condition" may be an average value of a plurality of measurements,
similarly to the third operational example in the second embodiment, or
alternatively, the latest value may be stored or the maximum or minimum
value within a certain period, or the like, may be stored.

[0299] After the handover decision (S19), the serving base station 200a
carries out transmission possible/not possible judgment (S76). For
example, similarly to the first operational example, the forwarding data
determination unit 243 or the handover decision unit 241 judges whether
or not it is possible to transmit the data awaiting transmission, by
finding out whether or not the predicted transmission time is less than
the maximum reservable time (S761) in the flowchart illustrated in FIG.
35.

[0300] Returning to FIG. 40, the serving base station 200a is able to
transmit the PDUs awaiting transmission (S77) if it is judged by the
transmission possible/not possible judgment (S76) that the PDUs awaiting
transmission can be transmitted. In the example in FIG. 40, the serving
base station 200a judges that the PDUs having sequence numbers SN5 and
SN6 which are awaiting transmission can be transmitted, and transmits
these PDUs. On the other hand, if it is judged that the data awaiting
transmission cannot be transmitted, then the serving base station 200a
does not transmit the data awaiting transmission.

[0301] Next, the serving base station 200a establishes handover (S78) and
transmits a handover request to the target base station 200b (S79).

[0302] The serving base station 200a then performs recovery of data
forwarding (S20) and determines the forwarding data based on the radio
quality status (S82).

[0303] The forwarding data determination process can be executed according
to the flowchart illustrated in FIG. 39, similarly to the second
operational example in the fifth embodiment, for instance. For example,
the forwarding data determination unit 243 determines the forwarding data
by using the radio wave condition table 233 stored in the memory unit
230.

[0304] When the forwarding data determination process starts (S810), the
forwarding data determination unit 243 judges the radio quality of the
handover destination cell (S811). Similarly to the third operational
example in the second embodiment, for example, the forwarding data
determination unit 243 is able to read out and assess the "quality
judgment" for the target base station 200b in the radio wave condition
table 233.

[0305] The forwarding data determination unit 243 sets the "SDU under
processing" and the "SDUs awaiting processing" as the forwarding data
(S814), if the radio quality of the handover destination cell is "poor"
("poor" at step S811). In the example in FIG. 33, if the radio quality is
"poor", then in the forwarding data, the "SDU under processing" is SDU-A
and the "SDUs awaiting processing" are SDU-B and SDU-C.

[0306] On the other hand, if the radio quality of the handover destination
cell is "good" ("good" at S811), then the forwarding data determination
unit 243 judges whether or not the predicted transmission time is less
than the maximum reservable time (S812). Similarly to the second
operational example, the forwarding data determination unit 243 judges
whether or not the PDUs awaiting transmission can be transmitted (or
whether or not the PDUs awaiting transmission is transmitted), for
instance. In the third operational example, if it is judged that the PDUs
awaiting transmission can be transmitted, in the transmission possible or
not possible judgment process (S76), then the PDUs awaiting transmission
are transmitted (S76), and if the PDUs awaiting transmission is
transmitted, then transmission of these PDUs to the target base station
200b can be omitted.

[0307] Consequently, if the predicted transmission time is less than the
maximum reservable time (Yes at S812), then the forwarding data
determination unit 243 sets the "SDUs awaiting processing" as the
forwarding data (S813). For example, if the PDUs having sequence numbers
SN5 and SN6 which are awaiting transmission can be transmitted (or is
transmitted), then the forwarding data determination unit 243 can
determine SDU-B and SDU-C, which are "SDUs awaiting processing", as the
forwarding data.

[0308] On the other hand, if the predicted transmission time is equal to
or greater than the maximum reservable time (No at S812), then the
forwarding data determination unit 243 sets the "SDUs under processing"
and the "SDUs awaiting processing" as the forwarding data (S814). For
example, if the PDUs having sequence numbers SN5 and SN6 which are
awaiting processing cannot be transmitted (or does not be transmitted),
then the forwarding data determination unit 243 can determine SDU-A,
which is an "SDU under processing", and SDU-B and SDU-C, which are "SDUs
awaiting processing", as the forwarding data. By means of the foregoing,
the serving base station 200a is able to determine the forwarding data.

[0309] Returning to FIG. 40, the serving base station 200a forwards the
forwarding data to the target base station 200b (S22). In this case,
similarly to the first operational example and the like, the serving base
station 200a may report the sequence number of the PDU from which
transmission is started in the target base station 200b (S24).

[0310] The serving base station 200a then transmits the downlink
allocation information (DL allocation) to the terminal 100 (S90), and the
terminal 100 carries out synchronization processing with the target base
station 200b, which is the handover destination (S91). The terminal 100
is able to receive the forwarded data from the base station 200b which
becomes the serving base station (S23).

[0311] <Fourth Operational Example>

[0312] Next, a fourth operational example, in other words, a combination
of the first to third operational examples will be described. Firstly, a
combination of the first operational example (the retransmission status
for each call) and the second operational example (the radio quality
based on the retransmission occurrence rate) will be described, whereupon
a combination of the first operational example and the third operational
example (radio quality based on the radio wave condition) will be
described, and finally a combination of the second operational example
and the third operational example will be described.

[0313] <1. Combination of First Operational Example and Second
Operational Example>

[0314] Firstly, a combination of the first operational example and the
second operational example according to the fifth embodiment will be
described. FIG. 41 to FIG. 43 are diagrams illustrating a sequence
example, and the like, according to this operational example. Parts which
perform the same processing as the first operational example and the
second operational example are labeled with the same reference numerals.

[0315] Of these, FIG. 41 is a diagram illustrating a sequence example, and
the like, according to this operational example. In respect of the
communication condition, similarly to the first operational example, and
the like, the serving base station 200a transmits PDUs having sequence
numbers SN1 to SN4 to the terminal 100, and of these, receives Ack
signals corresponding to the PDUs having sequence numbers SN1 and SN2.
Furthermore, the serving base station 200a makes a handover decision
before transmitting the sequence numbers SN5 and SN6 (S19), and therefore
the PDUs having sequence numbers SN5 and SN6 are awaiting transmission.

[0316] In a communication condition of this kind, similarly to the first
operational example, the serving base station 200a holds the
retransmission status for each call, and the transmission time, in the
retransmission information table 231 (see FIG. 34, for example) (S75).
For instance, the first data communication condition gathering unit 221
stores the retransmission status and the transmission time in the
retransmission information table 231.

[0317] Thereupon, the serving base station 200a decides to carry out
handover (S19) and, similarly to the second operational example, stores
the retransmission status and the retransmission occurrence rate for each
adjacent cell, in the statistical information table 232 (S30). For
instance, the second data communication condition gathering unit 242
stores values, or the like, in the respective items of the statistical
information table 232 (see FIG. 13A, for example), based on the
retransmission information table 231.

[0318] Thereupon, the serving base station 200a determines whether or not
it is possible to transmit the PDUs awaiting transmission (S76). For
example, similarly to the first operational example according to the
fifth embodiment, and the like, the handover decision unit 241 or
forwarding data determination unit 243 carries out a transmission
possible/not possible judgment process (see FIG. 35, for example) and
decides whether or not transmission is possible, by judging whether or
not the predicted transmission time is less than the maximum reservable
time (S761 in FIG. 35, for example).

[0319] The serving base station 200a transmits the PDUs awaiting
transmission (S77) if it is judged by the transmission possible/not
possible judgment (S76) that the PDUs awaiting transmission can be
transmitted. On the other hand, if it is judged by the transmission
possible/not possible judgment (S76) that the PDUs awaiting transmission
cannot be transmitted, then the serving base station 200a does not
transmit these PDUs.

[0320] Next, the serving base station 200a establishes handover (S78) and
transmits a handover request to the target base station 200b (S79).

[0321] The serving base station 200a then performs recovery of data
forwarding (S20) and determines the forwarding data based on the radio
quality status (S83).

[0322]FIG. 42 is a flowchart illustrating an example of a forwarding data
determination process according to the present operational example. For
example, the forwarding data determination unit 243 is able to determine
forwarding data by using the retransmission information table 231 (see
FIG. 34, for example) and the statistical information table 232 (see FIG.
13A, for example) which are stored in the memory unit 230.

[0323] Upon starting the forwarding data determination process (S830), the
forwarding data determination unit 243 judges the retransmission status
(S831). For example, similarly to the first operational example of the
fifth embodiment, the forwarding data determination unit 243 is able to
judge the retransmission status based on the presence or absence of the
retransmission, which is stored in the retransmission information table
231.

[0324] The forwarding data determination unit 243 sets the "SDU under
processing" and the "SDUs awaiting processing" as forwarding data, if the
judgment is "retransmission" for the terminal 100 which is being handed
over ("retransmission" at S831) (S835). For example, if the serving base
station 200a performs the retransmission in respect of the PDUs having
sequence numbers SN3 and SN4 in the example in FIG. 33, then it is
possible to determine the forwarding data as the "SDU under processing",
which is SDU-A that includes the PDUs having these sequence numbers, and
the "SDUs awaiting processing", which are SDU-B and SDU-C.

[0325] On the other hand, the forwarding data determination unit 243
judges the radio quality in the handover destination area (S832), when
there is a "no retransmission" judgment ("no retransmission" at S831) in
respect of the terminal 100 which is being handed over. The radio quality
is judged, for example, by means of the forwarding data determination
unit 243 reading out the "quality judgment" item in respect of the target
base station 200b in the statistical information table 232.

[0326] The forwarding data determination unit 243 sets the "SDU under
processing" and the "SDUs awaiting processing" as the forwarding data
(S835), if the radio quality of the handover destination area is "poor"
("poor" at step S832).

[0327] For example, even if the serving base station 200a does not be
carried out the retransmission in respect of the PDUs ("no
retransmission" at S831), when the radio quality in relation to the
target base station 200b is "poor" ("poor" at S833), then the possibility
that the terminal 100 correctly receives the PDU under processing is low
compared to a case where the radio quality is "good". Therefore, in a
case of this kind, the forwarding data determination unit 243 sets the
"SDU under processing" and the "SDUs awaiting processing" as the
forwarding data.

[0328] On the other hand, if the radio quality of the handover destination
area is "good" ("good" at S832), then the forwarding data determination
unit 243 judges whether or not the predicted transmission time is less
than the maximum reservable time (S833). Similarly to the judgment in the
first operational example (S802 in FIG. 37, for example), this judgment
involves determining whether or not it is possible to transmit the PDUs
awaiting transmission, for example, (or whether or not the PDUs awaiting
transmission could be transmitted).

[0329] Consequently, if the predicted transmission time is less than the
maximum reservable time (Yes at S833), then the forwarding data
determination unit 243 sets the "SDUs awaiting processing" as the
forwarding data (S834).

[0330] For example, if no retransmission is performed ("no retransmission"
at S831) and the radio quality is "good" ("good" at S832), and if the
PDUs awaiting transmission can be transmitted (or is transmitted) (Yes at
S833), then the possibility that the PDUs awaiting transmission is
correctly received in the terminal 100 is higher than cases where the
radio quality is "poor". In cases such as this, the forwarding data
determination unit 243 can omit the SDU including the PDUs awaiting
transmission (or the "SDU under processing") from the forwarding data. By
omitting the SDU including PDUs awaiting transmission, from the
forwarding data, it is possible to prevent situations where the PDUs
awaiting transmission are transmitted to the terminal 100 from the target
base station 200b despite the fact that the PDUs awaiting transmission is
transmitted (S76), and therefore duplicated transmission and duplicated
reception can be prevented. In the example in FIG. 33, if no
retransmission is performed, if the radio quality in relation to the
target base station 200b is "good", and if the PDUs having sequence
numbers SN3 and SN4 which are awaiting transmission is transmitted (S77),
then the forwarding data is SDU-B and SDU-C, which are the "SDUs awaiting
processing".

[0331] On the other hand, if the predicted transmission time is equal to
or greater than the maximum reservable time (No at S833), then the
forwarding data determination unit 243 sets the "SDUs under processing"
and the "SDUs awaiting processing" as the forwarding data (S835).

[0332] For example, if no retransmission is performed ("no retransmission"
at S831), if the radio quality is "good" ("good at S832), and if the PDUs
awaiting transmission cannot be transmitted (Yes at S833), then the PDUs
awaiting transmission are not transmitted from the serving base station
200a to the terminal 100. In cases such as this, in order to prevent data
loss in the terminal 100, the forwarding data determination unit 243 sets
the "SDU under processing" which includes PDUs awaiting transmission and
the "SDUs awaiting processing" as the forwarding data.

[0333] By means of the foregoing, a forwarding data determination process
(S83 in FIG. 41) is carried out, and the serving base station 200a then
forwards the forwarding data determined by the forwarding data
determination process (S22). In this case, similarly to the first
operational example and the like, the serving base station 200a may
report the sequence number of the PDU from which transmission is to be
started (S24).

[0334]FIG. 43 is a diagram illustrating an example of forwarding data in
the example in FIG. 41. As illustrated in FIG. 43, if the retransmission
status is "no retransmission", the radio quality is "good" and there are
no untransmitted PDUs (the PDUs awaiting transmission is transmitted
(S77) or can be transmitted), then the sequence number SN7 is reported.
On the other hand, if the retransmission status is "no retransmission",
the radio quality is "good" and there are untransmitted PDUs (the PDUs
awaiting transmission is transmitted or cannot be transmitted), then the
sequence number SN5 of the PDU awaiting transmission is reported. In
other situations, the sequence number SN3 of the PDU under transmission
for which an Ack signal does not be received, is reported.

[0335] The serving base station 200a then transmits the downlink
allocation information (AL allocation) to the terminal 100 (S90), and the
terminal 100 carries out synchronization processing with the target base
station 200b, which is the handover destination (S91), and is able to
receive the forwarded data (S23).

[0336] <2. Combination of First Operational Example and Third
Operational Example>

[0337] Next, a fourth operational example is described, which is an
operational example that combines the first operational example (the
retransmission status for each call) and the third operational example
(the radio quality based on the radio wave status). FIG. 44 is a diagram
illustrating a sequence example according to this operational example.

[0338] Similarly to the first operational example, the serving base
station 200a holds the retransmission status for each call, and the
transmission time, in the retransmission information table 231 (see FIG.
34, for example) (S75). Furthermore, similarly to the third operational
example, the serving base station 200a holds the radio wave condition in
the radio wave condition table 233 (see FIG. 17A, for example) (S35).

[0339] Next, the serving base station 200a decides to carry out handover
(S19), and carries out transmission possible/not possible judgment (S76).
The serving base station 200a is able to transmit the PDUs awaiting
transmission to the terminal 100 (S77), if it is determined by the
transmission possible/not possible judgment (S76 in FIG. 35, for example)
that the PDUs awaiting transmission can be transmitted. On the other
hand, the serving base station 200a does not transmit the PDUs awaiting
transmission if it is judged by the transmission possible/not possible
judgment (S76) that the PDUs awaiting transmission cannot be transmitted.

[0340] Next, the serving base station 200a establishes handover (S78) and
transmits a handover request to the target base station 200b (S79).

[0341] The serving base station 200a then performs recovery of data
forwarding (S20) and determines the forwarding data based on the radio
quality status (S84). The forwarding data determination process according
to this operational example can be implemented by the flow in FIG. 42,
similarly to the combination of the first operational example and the
second operational example, for instance. In this case, the radio quality
of the handover destination area in FIG. 42 is judged by the "good" or
"poor" value stored in the "radio quality" item of the radio wave status
table 233.

[0342] In the forwarding data determination process according to the
present operational example also, if the retransmission status is "no
retransmission" ("no retransmission" at S831), the radio quality is
"good" ("good" at S832), and the PDUs awaiting transmission can be
transmitted (or is transmitted) (Yes at S833), then the "SDUs awaiting
processing" are set as forwarding data (S834). By this means, for
example, it is possible to prevent duplicated transmission and duplicated
reception.

[0343] On the other hand, in other circumstances relating to the
retransmission status, the radio quality and the PDUs awaiting
transmission ("retransmission" at S831, "poor" at S832 or "No" at S833),
then the "SDU under processing" and the "SDUs awaiting processing" are
set as the forwarding data (S835). By this means, for example, it is
possible to prevent data loss.

[0344] Returning to FIG. 44, the serving base station 200a transmits the
forwarding data determined by the forwarding data determination process
(S84), to the target base station 200b (S22). In this case, the serving
base station 200a is also able to report the sequence number (S24). FIG.
43 illustrates an example of a sequence number which is reported in the
present operational example, similarly to an example where the first
operational example and the second operational example are combined.

[0345] Thereafter, the serving base station 200a transmits allocation
information (DL allocation) (S90), and the terminal 100 carries out
synchronization processing (S91) and is able to receive forwarding data
from the base station 200b which becomes the serving base station (S23).

[0346] <3. Combination of Second Operational Example and Third
Operational Example>

[0347] Next, a combination of the second operational example (radio
quality based on the retransmission occurrence rate) and the third
operational example (radio quality based on radio wave status) will be
described. FIG. 45 and FIG. 46 respectively illustrate sequence examples
according to the present operation.

[0348] Similarly to the first or second operational example, the serving
base station 200a holds the retransmission status for each call, and the
transmission time, in the retransmission information table 231 (see FIG.
34, for example) (S75). Furthermore, similarly to the third operational
example, the serving base station 200a holds the radio wave condition in
the radio wave condition table 233 (see FIG. 17A, for example) (S35).

[0349] Thereupon, when the serving base station 200a decides to carry out
handover (S19), similarly to the second operational example, it stores
the retransmission status and the retransmission occurrence rate for each
adjacent cell, in the statistical information table 232 (S30). For
instance, the second data communication condition gathering unit 242
stores values, or the like, in the respective items of the statistical
information table 232 (see FIG. 13A, for example), based on the
retransmission information table 231.

[0350] Thereupon, the serving base station 200a decides whether or not the
PDUs awaiting transmission can be transmitted (S76), transmits the PDUs
awaiting transmission if the PDUs awaiting transmission can be
transmitted according to the transmission possible/not possible judgment
(S76), and does not transmit these PDUs if they cannot be transmitted.

[0351] Next, the serving base station 200a establishes handover (S78) and
transmits a handover request to the target base station 200b (S79).

[0352] The serving base station 200a then performs recovery of data
forwarding (S20) and determines the forwarding data (S85). Similarly to
the fourth embodiment, the forwarding data determination process
according to this operational example determines the final "radio
quality" based on a combination of two radio qualities, namely, the
"radio quality" in the statistical information table 232 and the "radio
quality" in the radio wave condition table 233 (see FIG. 32, for
example). Processing is then implemented by applying the determined
"radio quality" as the "radio quality of the handover destination cell"
(S811 in FIG. 39) in the forwarding data determination process (see FIG.
39, for example).

[0353] For example, as illustrated in FIG. 32, if the "radio quality" in
the statistical information table 232 (the radio quality based on the
retransmission occurrence rate" in FIG. 32) is "good" and the "radio
quality" in the radio wave condition table 233 (the "radio quality based
on the radio wave condition" in FIG. 32) is "good", then the final radio
quality can be determined as "good". In this case, in the forwarding data
determination process (FIG. 39, for example), the radio quality of the
handover destination cell is judged to be "good" ("good" at S811). On the
other hand, if the "radio quality" of the two tables 232 and 233 is not
"good" in both cases, then the final radio quality is judged to be "poor"
(see FIG. 32, for example), and the radio quality of the handover
destination cell is judged to be "poor" ("poor" at S811).

[0354] In the operational examples below, similarly to the fifth
embodiment, the forwarding data can be determined by means of a
forwarding data determination process (see FIG. 39, for example).

[0355] Returning to FIG. 45, when the forwarding data is determined (S85),
the serving base station 200a is able to carry out data forwarding (S22
in FIG. 46) and also report a sequence number (S24). The processing
thereafter can be carried out similarly to the third operational example,
or the like.

Sixth Embodiment

[0356] Next, a sixth embodiment of the invention will be described. The
fifth embodiment was described with reference to an example where the
forwarding data is determined after transmitting PDUs awaiting
transmission, when there are PDUs awaiting transmission. The sixth
embodiment is described in relation to an example where the forwarding
data is determined in advance, and PDUs awaiting transmission are
transmitted subsequently. FIG. 47 to FIG. 55 are diagrams which
respectively illustrate sequence examples, and the like, according to the
sixth embodiment.

[0357] The respective compositional examples of the radio communication
system 10, the base stations 200a and 200b, and the terminal 100, are
similar to the second embodiment, for instance, which are illustrated
respectively in FIG. 2 to FIG. 4. Furthermore, the communication
conditions and the compositional examples of the SDUs and PDUs, and the
like, are similar to the second embodiment.

[0358] The operational example according to the sixth embodiment includes
the following four patterns, similarly to the second embodiment or the
fifth embodiment.

[0359] 1) When forwarding data is determined based on the retransmission
status which is held for each call;

[0360] 2) When forwarding data is determined based on a data communication
condition, such as the retransmission occurrence rate which is held for
each adjacent cell;

[0361] 3) When forwarding data is determined based on the radio wave
condition between the serving base station 200a and the terminal 100; and

[0362] 4) A combination of 1) to 3) above.

[0363] Below, four operational examples (first to fourth operational
examples) are described successively, similarly to the second embodiment.

[0364] <First Operational Example>

[0365] The first operational example is an example where, for instance, a
handover decision is made when there is data awaiting transmission, and
furthermore the forwarding data is determined based on the retransmission
status. FIG. 47 is a diagram illustrating a sequence example of a first
operational example according to the sixth embodiment. Processes which
are the same as the first operational example of the fifth embodiment,
and the like, are labeled with the same reference numerals.

[0366] In this sixth embodiment, the serving base station 200a carries out
transmission possible/not possible judgment (S76) and determines the
forwarding data (S80). For example, the serving base station 200a is able
to carry out the same processing as the fifth embodiment (see FIG. 35 and
FIG. 37, for example) in relation to the transmission possible/not
possible judgment process and the forwarding data determination process.

[0367] The serving base station 200a forwards the forwarding data (S22)
which is determined by the forwarding data determination process (S80),
and then transmits PDUs awaiting transmission (S77) if it is decided to
transmit the PDUs awaiting transmission by the transmission possible/not
possible judgment (S76).

[0368] Here, the forwarding data determination unit 243 (or the handover
decision unit 241) judges whether or not the predicted transmission time
is less than the maximum reservable time, in relation to the transmission
possible/not possible judgment process (S76, see FIG. 35 for example),
but the maximum reservable time is different to that of the fifth
embodiment. For example, in FIG. 36A or FIG. 36B, the maximum reservable
time (*2) is from the handover decision (S19) until immediately before
handover is established (S78), similarly to the fifth embodiment, but the
handover is established by the transmission of downlink allocation
information. The downlink allocation information includes identification
information relating to the handover destination base station, for
example, and therefore in the sixth embodiment, the serving base station
200a establishes handover by transmitting this allocation information.

[0369] For example, similarly to the second embodiment, the maximum
reservable time can be stored in the memory unit 230, or the like, and
read out as and when appropriate by the transmission possible/not
possible judgment process (S76) or a forwarding data determination
process (S80), or the like.

[0370] Furthermore, similarly to the fifth embodiment, the predicted
transmission time (*1) is the time from the handover decision until the
end of transmission of the PDUs awaiting transmission, and this time can
be calculated by the forwarding data determination unit 243, or the like,
based on the number of PDUs awaiting transmission and the transmission
time.

[0371] In the sixth embodiment, the transmission possible/not possible
judgment process (see FIG. 35, for example) can be implemented similarly
to the fifth embodiment, apart from the fact that the maximum reservable
time is different. Furthermore, the forwarding data determination process
(FIG. 37, for example) can be implemented similarly to the fifth
embodiment, apart from the fact that the maximum reservable time is
different.

[0372] If the retransmission is occurred, for example ("retransmission" at
S801 in FIG. 37), then SDU-A which includes a PDU having sequence number
SN3 can be set as the forwarding data. Furthermore, even if no
retransmission is occurred, if the data awaiting transmission cannot be
transmitted (No at S802 in FIG. 37), then SDU-A which includes the PDU
having sequence number SN5 can be set as the forwarding data. Moreover,
if no retransmission is occurred and if the data awaiting transmission
can be transmitted (Yes at S802 in FIG. 37), then SDU-B which includes
the PDU having sequence number SN7 can be set as the forwarding data. The
serving base station 200a can also report the sequence number determined
in this way, to the target base station 200b (S24).

[0373] Thereupon, the serving base station 200a establishes handover
(S78), and transmits downlink allocation information to the terminal 100
(S90). Thereafter, the processing is the same as the fifth embodiment.

[0374] In this first operational example, after a handover decision (S19),
the serving base station 200a recovers data forwarding (S20), and then
transmits a handover request to the target base station 200b. The
handover request can be transmitted between the handover decision (S19)
and the recovery of data forwarding (S20), for example.

[0375] <Second Operational Example and Third Operational Example>

[0376] The sixth embodiment differs from the fifth embodiment in that, as
described in the first operational example above, a forwarding data
determination process is carried out (S80 in FIG. 47) and then data
awaiting transmission is transmitted (S77), but the processing apart from
this is virtually the same as the fifth embodiment.

[0377]FIG. 48 and FIG. 49 respectively illustrate sequence examples
according to the second operational example, for instance. Similarly to
the second operational example of the fifth embodiment, the serving base
station 200a holds the retransmission status (or the presence or absence
of the retransmission), and the transmission time, in the retransmission
information table 231 (S75), calculates the retransmission occurrence
rate, and the like, and stores this information in the statistical
information table 232 (S30).

[0378] The serving base station 200a judges whether or not transmission is
possible in respect of the PDUs awaiting transmission (S76), and
determines the forwarding data based on the statistical information table
232 (S81). The transmission possible/not possible judgment process and
the forwarding data determination process can also be carried out
similarly to the second operational example of the fifth embodiment (see
FIG. 35 and FIG. 39, for example).

[0379] However, similarly to the first operational example, the serving
base station 200a establishes handover (S78) immediately before
transmitting the downlink allocation information, and the maximum
reservable time is a different time to that of the fifth embodiment.

[0380] The serving base station 200a transmits the forwarding data to the
target base station 200b (S22), and if the PDUs awaiting transmission can
be transmitted, transmits these PDUs (S77). Thereupon, the serving base
station 200a establishes handover (S78 in FIG. 49), and transmits
downlink allocation information (DL allocation) to the terminal 100.
Thereafter, the processing carried out is the same as the first
operational example in the sixth embodiment.

[0381] Next, a third operational example will be described. FIG. 50 is a
diagram illustrating a sequence example according to the third
operational example. The serving base station 200a saves the radio wave
condition in the radio wave condition table 233 (S35), and after deciding
whether or not transmission is possible (S76), determines the forwarding
data based on the saved radio wave condition table 233 (S82).

[0382] The serving base station 200a transmits the forwarding data to the
target base station 200b, and if the PDUs awaiting transmission can be
transmitted, transmits these PDUs (S77). Thereupon, the serving base
station 200a establishes handover (S78), and transmits downlink
allocation information (DL allocation) to the terminal 100 (S90).
Thereafter, the processing carried out is the same as the first
operational example in the sixth embodiment.

[0383] <Fourth Operational Example>

[0384] Next, operational examples which combine the first to third
operational examples will be described. In these respective cases, the
processing differs from the fifth embodiment in that a forwarding data
determination process is carried out (S83 in FIG. 51, for example),
whereupon the data awaiting transmission is transmitted (S77), and the
processing apart from this is virtually the same as the fifth embodiment.

[0385]FIG. 51 and FIG. 52 are diagrams illustrating sequence examples of
a case where a first operational example (retransmission status for each
call) and a second operational example (radio quality based on
retransmission occurrence rate) are combined. Similarly to the fourth
operational example of the fifth embodiment, the serving base station
200a holds the retransmission status (or the presence or absence of the
retransmission), and the transmission time, in the retransmission
information table 231 (S75), calculates the retransmission occurrence
rate, and the like, and stores this information in the statistical
information table 232 (S30).

[0386] The serving base station 200a judges whether or not transmission is
possible in respect of the PDUs awaiting transmission (S76), and
determines the forwarding data based on the retransmission status and the
radio quality, and the like (S83). FIG. 42 illustrates a sequence example
of a forwarding data determination process in this fourth operational
example, for instance, and apart from the fact that the maximum
reservable time differs from that of the fifth embodiment, processing can
be implemented similarly to the fifth embodiment.

[0387] After determining the forwarding data, the serving base station
200a transmits the forwarding data to the target base station 200b (S22),
and if it is decided by the transmission possible/not possible judgment
(S76) that the PDUs awaiting transmission can be transmitted, then these
PDUs are transmitted to the terminal 100 (S77).

[0388] Thereupon, the serving base station 200a establishes handover (S78
in FIG. 52), and transmits downlink allocation information (DL
allocation) to the terminal 100 (S90). Thereafter, the processing carried
out is the same as the first operational example in the sixth embodiment.

[0389]FIG. 53 is a diagram illustrating a sequence example of a case
where the first operational example (retransmission status for each call)
and the third operational example (radio quality based on radio wave
condition) are combined. Similarly to the fourth operational example of
the fifth embodiment, the serving base station 200a holds the
retransmission status (or the presence or absence of the retransmission),
and the transmission time, in the retransmission information table 231
(S75), and stores the radio wave condition in the radio wave condition
table 233 (S35).

[0390] The serving base station 200a judges whether or not transmission is
possible in respect of the PDUs awaiting transmission (S76), and
determines the forwarding data based on the retransmission information
table 231 and the radio wave condition table 233, and the like (S84). The
forwarding data determination process can be implemented in the same
manner as the fourth operational example according to the fifth
embodiment (see FIG. 42, for example).

[0391] After determining the forwarding data, the serving base station
200a transmits the forwarding data to the target base station 200b (S22),
and if it is decided by the transmission possible/not possible judgment
(S76) that the PDUs awaiting transmission can be transmitted, then these
PDUs are transmitted to the terminal 100 (S77). Thereupon, the serving
base station 200a establishes handover (S78), and transmits downlink
allocation information (DL allocation) to the terminal 100 (S90).
Thereafter, the processing carried out is the same as the first
operational example in the sixth embodiment.

[0392]FIG. 54 and FIG. 55 are diagrams illustrating sequence examples of
a case where the second operational example and the third operational
example are combined. Similarly to the fourth operational example of the
fifth embodiment, the serving base station 200a holds the retransmission
status (or the presence or absence of the retransmission), and the
transmission time, in the retransmission information table 231 (S75), and
stores the radio wave condition in the radio wave condition table 233
(S35). Furthermore, the serving base station 200a calculates the
retransmission occurrence rate based on the retransmission information
table 231 and holds the retransmission status, the retransmission
occurrence rate, and the like, in the statistical information table 232
(S30).

[0393] The serving base station 200a judges whether or not transmission is
possible in respect of the PDUs awaiting transmission (S76), and
determines the forwarding data based on the statistical information table
232 and the radio wave condition table 233, and the like (S65).
Thereafter, the processing carried out is the same as the first
operational example in the sixth embodiment.

[0394] First to fourth operational examples relating to the sixth
embodiment were described above, but similarly to the fifth embodiment,
the serving base station 200a according to the sixth embodiment also
transmits data awaiting transmission to the terminal 100, if there is
data awaiting transmission and this data can be transmitted. The serving
base station 200a does not forward the data awaiting transmission to the
target base station 200b.

[0395] By this means, data awaiting transmission (for example, PDUs having
sequence numbers SN5 to SN6) is not transmitted to the terminal 100 from
the target base station 200b, and the target base station 200b does not
transmit the data awaiting transmission to the terminal 100 in a
duplicated fashion, as well as the serving base station 200a.
Furthermore, in this case, the terminal 100 does not receive the data
awaiting transmission (for example, sequence numbers SN5 to SN6) from two
base stations 200a and 200b, and hence there is no duplicated
transmission.

[0396] Moreover, since the data awaiting transmission (for example,
sequence numbers SN5 to SN6) is transmitted from the serving base station
200a (for example, in step S77), then it is possible to avoid situations
where the data awaiting transmission is not transmitted and a data loss
occurs.

Seventh Embodiment

[0397] In the second embodiment described above,

1) In a first operational example, for instance, forwarding data is
determined based on the retransmission status (presence or absence of the
retransmission) which is held by the serving base station 200a for each
call. 2) Furthermore, in a second operational example, for instance,
forwarding data is determined based on a data communication condition,
such as the retransmission occurrence rate, which is held by the serving
base station 200a for each adjacent cell. 3) Moreover, in a third
operational example, forwarding data is determined based on a radio wave
condition between the serving base station 200a (for example, the
handover source base station) and the terminal 100.

[0398] The second embodiment is described with respect to an example of a
combination of the first operational example and the second operational
example, and an example of a combination of the first operational example
and the third operational example.

[0399] Furthermore, in the fourth embodiment, a combination of the second
operational example and the third operational example is also described.

[0400] In this seventh embodiment, an example of a combination of the
first operational example, the second operational example and the third
operational example is described. The sequence according to the seventh
embodiment can be carried out according to FIG. 31 which was described in
the fourth embodiment, for instance.

[0401] More specifically, similarly to the fourth operational example, the
serving base station 200a holds the retransmission status for each call,
in the retransmission information table 231 (S18). FIG. 6 illustrates an
example of the retransmission information table 231.

[0402] Furthermore, similarly to the fourth embodiment, the serving base
station 200a holds the radio quality which is measured at the terminal
100 or the serving base station 200a, as the radio wave condition, in the
radio wave condition table 233 (S35). FIG. 17A is a diagram illustrating
an example of a radio wave condition table 233.

[0403] Moreover, similarly to the fourth operational example, the serving
base station 200a stores the retransmission status and the retransmission
occurrence rate for each adjacent cell, in the statistical information
table 232 (S30). FIG. 13A illustrates an example of the statistical
information table 232.

[0404] The serving base station 200a judges the radio quality from the
held information and determines the forwarding data (S65).

[0405]FIG. 56 illustrates judgment examples of how the "radio quality" is
judged based on the combination of the three elements: the radio status
of each call, the radio wave condition, and the retransmission status of
each adjacent cell. For example, the retransmission status for each call
which is held in the retransmission information table 231 corresponds to
the "retransmission status of each call" in FIG. 56. Moreover, the radio
quality based on the retransmission occurrence rate held in the
statistical information table 232 corresponds to the "radio quality based
on retransmission occurrence rate" in FIG. 56. Furthermore, the radio
quality based on the radio wave condition held in the radio wave
condition table 233 corresponds to the "radio quality based on radio wave
condition" in FIG. 56.

[0406] For example, the forwarding data determination unit 243 of the
serving base station 200a respectively reads out the retransmission
status for each call, the radio quality based on the retransmission
occurrence rate and the radio quality based on the radio wave condition,
respectively from the three tables 231, 232 and 233 stored in the memory
unit 230. The forwarding data determination unit 243 judges that the
radio quality is "good" if, as illustrated in FIG. 56, for example, the
"retransmission status for each call" is "no" (=no retransmission), and
if the "radio quality based on retransmission occurrence rate" and the
"radio quality based on radio wave condition" are both "good". In any
other situation, for example, the forwarding data determination unit 243
judges that the "radio quality" is "poor".

[0407] The forwarding data determination unit 243 determines the
forwarding data by taking the judgment result for "radio quality" as the
judgment result for S301 in FIG. 9B, for example (S302 and S303). In this
case, if the "radio quality" is "good", then the forwarding data
determination unit 243 is able to set the "SDUs awaiting processing" (for
example, the SDUs from "SDU-B" onwards) as the forwarding data (S302,
FIG. 56). On the other hand, if the "radio quality" is "poor", then the
forwarding data determination unit 243 is able to set the "SDUs under
processing" (for example, "SDU-A" and the SDUs from "SDU-B" onwards) as
the forwarding data (S303, FIG. 56).

[0408] In this case, a final judgment of "good" for the "radio quality"
also takes account of the "retransmission status for each call", and
therefore the reliability can be raised (enhanced) in comparison with an
example where the second operational example and the third operational
example are combined (fourth embodiment).

[0409] An example which combines all of the first to third operational
examples can also be implemented in the fifth embodiment and the sixth
embodiment.

[0410] For example, FIG. 45 relating to the fifth embodiment illustrates a
sequence diagram of a case where there are untransmitted PDUs which does
not be transmitted to the terminal 100 before the handover decision, and
the forwarding data is determined (S85) after the untransmitted PDUs is
transmitted to the terminal 100 (S77). In the fifth embodiment, an
operational example which combines the first to third operational
examples can be implemented based on FIG. 45, for example.

[0411] More specifically, the serving base station 200a stores the
retransmission status and transmission time of each call in the
retransmission information table 231 (S75). FIG. 34 illustrates an
example of the retransmission information table 231.

[0412] Furthermore, the serving base station 200a holds the radio quality
which is measured at the terminal 100 or the serving base station 200a,
as the radio wave condition, in the radio wave condition table 233 (S35).
FIG. 17A is a diagram illustrating an example of a radio wave condition
table 233.

[0413] Moreover, the serving base station 200a then stores the
retransmission status and the retransmission occurrence rate for each
adjacent cell in the statistical information table 232 (S30). FIG. 13A
illustrates an example of the statistical information table 232.

[0414] The serving base station 200a judges the radio quality from the
held information and determines the forwarding data (S85). The forwarding
data can be determined similarly to the examples described above, as
illustrated in FIG. 56, for example. In this case, for instance, the
forwarding data determination unit 243 can determine the forwarding data
similarly to the fifth embodiment, by using the judgment results in S811
of FIG. 39 which illustrates an example of a forwarding data
determination process.

[0415] Moreover, FIG. 54 in the sixth embodiment, for instance,
illustrates a sequence diagram of a case where there are untransmitted
PDUs which does not be transmitted to the terminal 100 before the
handover decision, and the untransmitted PDUs are transmitted to the
terminal 100 (S77) after the forwarding data is determined (S65). In the
sixth embodiment, an operational example which combines the first to
third operational examples can be implemented based on FIG. 54, for
example.

[0416] More specifically, the serving base station 200a stores the
retransmission status and transmission time of each call in the
retransmission information table 231 (S75). FIG. 34 illustrates an
example of the retransmission information table 231.

[0417] Furthermore, the serving base station 200a holds the radio quality
which is measured at the terminal 100 or the serving base station 200a,
as the radio wave condition, in the radio wave condition table 233 (S35).
FIG. 17A is a diagram illustrating an example of a radio wave condition
table 233.

[0418] Moreover, the serving base station 200a then stores the
retransmission status and the retransmission occurrence rate for each
adjacent cell in the statistical information table 232 (S30). FIG. 13A
illustrates an example of the statistical information table 232.

[0419] The serving base station 200a judges the radio quality from the
held information and determines the forwarding data (S65). The forwarding
data can be determined similarly to the examples described above, as
illustrated in FIG. 56, for example. In this case, for instance, the
forwarding data determination unit 243 can determine the forwarding data
similarly to the fifth embodiment, by using the judgment result from S811
of FIG. 39, which illustrates an example of a forwarding data
determination process.

[0420] In the fifth and sixth embodiments, an operational example which
combines the first to third operational examples takes account of the
"radio status of each call" in the final judgment of "radio quality", and
therefore the reliability can be improved further in comparison with an
example where the second and third operational examples are combined.

[0421] All examples and conditional language recited herein are intended
for pedagogical purposes to aid the reader in understanding the invention
and the concepts contributed by the inventor to furthering the art, and
are to be construed as being without limitation to such specifically
recited examples and conditions, nor does the organization of such
examples in the specification relate to a showing of the superiority and
inferiority of the invention. Although the embodiment(s) of the present
invention has been described in detail, it should be understood that the
various changes, substitutions, and alterations could be made hereto
without departing from the spirit and scope of the invention.